Yes, people use Steam because they can make pricing competitive enough (low) that they will find users that will pay for the game instead of just pirating it or trying to buy it somewhere else cheaper.
Steam also has built-in market arbitration, since they do not charge the same in all countries and you can buy keys that are still valid worldwide, people who find some games still too expensive will go and buy a key from a third-party seller and still play the game on Steam.

From the point of view of devs it is very worth it, because even though steam might take as big of a cut as Apple (at first, and debatable) this is on what wouldn't be sales otherwise so it ends up being better in the long run.
On top of that, Steam actually lowers the cut they take if your game is successful, which makes a lot more sense than what Apple does (if you make more than a 1m you are back to 30% which makes zero sense, because the more your product is successful the less relevancy Apple has in your success...).

And it seems pretty clear that those are just what is publicly available information, considering the number of latitudes/tools and options Steam give to developers it is obvious that most even moderately successful developers get to negotiate their terms. Unlike Apple they are willing to work on a case-by-case basis, instead of going the route of a giant soulless corp that will treat everyone the same, because the rules are the rules or whatever (all the while working out deals with actually powerful corporations in the back, still maintaining that extremely wrong do-gooder better than you attitude they have).

Another reason Steam is very different is that they are not exclusive, you can very well distribute both on Steam or somewhere else cheaper, including your own solution with no issue at all (many games do this). And it means that sales on Steam maybe wouldn't have happened somewhere else, which makes the cut feel a lot fairer.

So here you have it, Steam is very different in many ways, first and foremost because it is not your only option for the platform and they cannot have monopolistic behavior. But also, even though I think it is still a big corp with many issues, they are less of a pain in the ass to work with than Apple.

I agree that most people would still primarily use Apple's App Store, but this is exactly the point and the whole reason their arguments are irrelevant and anti-competitive. If people were to start using some other solution in large numbers, it would mean that Apple is indeed doing something very wrong.
And I do believe that they are way too greedy, on top of being way too controlling of the devices/systems they do not have ownership anymore. All their arguments about privacy and security are horseshit because if we accept them, it means private property is a rather vague concept and there would be no reason they get to keep their intellectual property at this point.

As for what you said about Android, I find it funny because just a few days ago I helped a friend install an ad-blocker from F-Droid to play a Tetris game (that has an insane 3euros/months subscription if you want to get rid of ads). So yeah, the majority of people use Google Play, but also the majority of people mostly use popular social media, a few utilities (cheap or free mostly) and some games. The Apple tax doesn't even apply to most cases because an app making a lot of revenue can figure out a way to make users pay in-app with no Google tax and the solution of their choosing.

And you are talking about Apple lowering rates to 10-15% and I think it seems acceptable because payments seem to be weirdly expensive in the US (because of credit card and cashback programs I have been told). But in the EU, it still seems like an absurdly high tax / cut for what is a payment solution. If you are dealing with micro-transaction (in the cents amounts) it could make sense, but with the app pricing inflation, it is largely not the case and you are talking about multiple euros amount most of the time. Payment solutions for this kind of transaction would work out to about 7% in the worst-case scenario, and mostly depending on the profile of customers, many devs could get rates as low as 3-4% if they have large volume.
If you think about the market as a whole, it is a huge amount of money, which is exactly the reason Apple is making so much of it.

Since I know, some are going to argue about Apple making their dev tools available and their distribution infrastructure, I will cover this very easily.
They need their dev tools for their own apps and the reason you have to use them is because they make it almost impossible to use anything else; on top of that without this their platform would be extremely uninteresting and completely irrelevant compared to Android, because who would buy a smartphone (that is just a small computer) that can only run apps from its manufacturer (they were called feature phones before...). Before Apple launched the App Store, people figured out how to make and publish apps without them just fine, without much need of Apple tooling. So, if the system was accessible as any computer system should be, the value of their tools would be almost non-existent in fact or a very low amount compared to what they take. Anyone that isn't a complete hypocrite knows they make those tools to make it easier and desirable to develop for their platform, they have a big interest in it, and every developer already paid for it when they bought their extremely expensive hardware (but modern Apple has no issues double-dipping everywhere).

As for the distribution, I actually think they should charge for it, even the "free" apps (I would say ESPECIALLY). There is no reason a small dev would have to give Apple at least 15% of all of his revenue for distribution when Facebook can distribute their app for free even though it must use an extremely large amount of bandwidth.
This would have many side effects: first the apps would become slimmer (since peoples start caring when they have to pay, even more for corporations, you can be certain they would they find ways to reduce the bandwidth bill), second there would be less updates (and that in itself would be an achievement) and third it would automatically cleanup the store since devs wouldn't publish trashy/scummy apps if they had to pay for the waste of ressource they cause (can't give back the time stolen from users anyway).

In any case, I believe that those "app stores" are a modern problem created by network effect inherent to technology, and it just isn't right to let companies run a very large tax rate on economics activities happening on their platforms. Apple is being targeted more because they are by far the worst of the bunch and also the richest target. There is often a lot of talk about market value, but it is irrelevant since it can be wiped pretty fast. The fact is Apple has a mountain of cash, and they got there by stealing more than their fair share from society.