I would love to be corrected and there is a lot of disinformation, at least from one side, but here is my understanding of the issue:

Let's say an ISP A wants to cover a small area with 1000 potential users. So, lets say they allocate 100gbps bandwidth for the market. So, they sell 100mbps internet to 1000 users. All of the users get 100mbps dedicated but it is expensive for them. Besides, not all users are using 100mbps all the time anyway. So, a competing ISP B could just allocate 10gbps (one-tenth of what other had) saving a lot of costs, and offer 100mbps at half the price with a condition that speeds could be lower at peak hours because if everyone is using full bandwidth (very unlikely), they will only get 1/10th of the advertised speeds (in practice, it's usually close to advertised speed most of the time). The ISP A is completely outpriced so they can either lower their prices or go out of business... and obviously they choose a similar strategy to be competitive.

This has been the case for a long time. They have optimized it with dynamic contention ratios and the like but this is basically how it works. Anyone who needs dedicated bandwidth knows that prices for those for the same speeds are very expensive compared to regular internet.

The problem with this is obviously "the problem users". Back in the days, they were the ones who would hog up maximum bandwidth most of the time using it for file sharing (seed/download all day long). In the above example, if 100 users use all the available bandwidth at all times, whole of the bandwidth is taken up all the time. So, during peak hours, when some of the other 900 joined in, the speeds were much slower than advertised.

The ISPs noticed that file sharing was using most of the bandwidth, so if they blocked it, they could improve internet experience of everyone (except for the pirates, which made a small percentage of their customer base). This is what Comcast did in 2007 by blocking bittorrent traffic. The pirates however, who also happened to be the most active and knowledgable internet users, were not happy obviously. So, two lobbies filed a case against Comcast making it a freedom issue (see Comcast Corp. v. FCC) which got a huge attention from the most active and vocal pirates.

This meant that ISPs could not differentiate between what kind of and how much traffic went through. This meant companies could offer services requiring very high bandwidth (eg. HD video streaming) and if the internet was slow, the ISPs would be the culprit (for advertising higher speeds than they were able to provide if a lot of users used such services).

Obviously, at this point, the best option for ISPs is to make internet more expensive. However, they obviously want to expand the market and rise in price could mean many users may just drop the service. So, they are going for this another option... which would allow them to charge different rates for profitable high-bandwidth using services and hence the cost would be transferred to the providers of those services and not the end customers. Obviously, those service providers (Google, Netflix, Pornhub etc.) don't want the cost to come to them (which might mean that their users would drop if they priced their services higher or entire business models to fail), they'd rather it be transferred to end users. So, they are vehemently pushing the "internet freedom"/"net neutrality" narrative when it's definitely not.

To answer your question, ISPs would be free to not use the fast/slow lane approach if they can be competitive with it.