As long as my app gains new users every day, new people will buy my app.

That works as long as there is plenty of market opportunity left for your app, which might be the case for a little business in a big market. It's not the case if you actually do have close to a 100% installed base in your target market, though, and that is what some of these huge software companies are facing.

What I don't buy is the idea that going subscription-only is the only plausible alternative. I agree with the author here that trying to force subscription-only pricing is often a warning sign of stagnation, precisely because I disagree with this:

Paid upgrades won’t help.

Why not? If you're providing useful new features, or relevant compatibility changes as other parts of the technology world evolve, or effective security fixes in the face of new threats, you have added value and you can offer that through paid upgrades.

I would happily pay a sensible rate to Microsoft to continue providing security and compatibility fixes for my various Windows 7 PCs for as long as I need them. I think it's reasonable to expect a certain amount of support to be included for free with something like an OS -- after all, those security vulnerabilities are by definition defects in the original software -- but it's not reasonable to expect indefinite free support, and I'd be open to a transparent and reasonably priced alternative model. However, I won't pay them for Windows 10 under any circumstances, because the forced updates and questionable privacy/security details are deal-breakers no matter what else it has going for it.

Even without the security and compatibility issues, there are plenty of functions Adobe could have added or improved in various Creative Suite applications that would have justified a paid upgrade for everyone who uses it in a small business I work with, and we would gladly have paid the kind of upgrade prices they used to offer for those improvements. Sadly, they never did make most of them despite numerous users apparently sharing our interest in them, so they never got our money for any upgrades. Given that so far we've noticed only a few modest improvements we would actually have cared about in all the time since they went to Creative Cloud, we're perfectly happy with our older CS releases and not being locked into paying them money for whatever they think is a worthy improvement instead of whatever we think is a worthy improvement. We're currently far more interested in some of the newer competitors springing up in various niches, some of which are or might become better tools for us anyway.

So I agree with the author that trying to force a subscription model with no apparent benefits to the user is usually a sign of commercial desperation, and I expect that at some point (probably relatively soon, by business standards) the market will push back and the companies trying to do this will suffer for it, though no doubt they'll make a substantial amount of extra money exploiting their pseudo-monopoly status for a while first. However, I don't agree that selling fresh copies to new customers and giving free upgrades is necessarily the only viable alternative.