This. The software would get abandoned once they've cashed in on the bulk of people buying it. They can trickle in more sales over time, but that's not going to amount to nearly as much as the initial payload, and therefore the motivation to continue developing dwindles.
You can release new versions though. This is what a large chunk of the software industry has done up until recently. Photoshop has gone through a lot of version numbers through the years.
Although it does sort of put pressure on the developers to actually refine their product and continue to deliver tangible improvements. Nobody is going to pay for a UI that's been re-arranged a bit.
Except a "new version" is arbitrary, and the decision to do so was made for financial reasons and not technical. Every new version of Photoshop had some percentage of people complaining "why can't this update just be a patch?".
Yeah sure, although upgrading was optional. My point is the incentive is on the developer to make it worth while to pay for an upgrade, they can't just phone it in.
There's even more pressure on a subscription service to deliver value, since you can cancel and the business makes less than selling MSRP (and this usually means losing money on the customer after customer acquisition costs).
I can't think of a single subscription service I use that doesn't continue to deliver new updates and value on a regular basis, and other than Adobe's bullshit "annual masquerading as monthly" drama I haven't come across something I wished was still packaged.