Maybe. But to be honest, plain Markdown isn’t very special. But a good tool could help users to solve common problems that happen if you write more than small how-tos in this plain and easy format.
Ulysses does not offer any of these features like e.g. assistants for creating indexes or glossaries, which is a bit challenging for developers, because page numbers depend on rendering — but that’s something for what users really would pay as they cannot get this from a plain text editor, while markdown itself could be written even with the most simple editor available.
Ulysses could have made also improvements with broader support of markdown dialects. But nothing happened during the years. Also management functions and better (optional) inline presentation of media assets could be worth money.
Not to say that the makers had — at any time — the opportunity to offer paid updates to the users. But they didn’t. Not a fault of the user — or is it?
Time will tell us, if this change is a success model — but if you ask me, i wouldn’t bet a cent, if they do not act. Yet more and more people discover that writing texts (not letters) is much more fun and less stress with easy to learn and use lightweight-formats and conventions. More and more apps appear on the market and there are some, which could make live really hard for the makers of Ulysses. Not forcibly because of the price, but with much better communication and more clear commitment and roadmap.
But to be honest, the whole subscription economy is IMHO doomed to fail, because the money of the people is the limit. And the more companies switch to subscriptions, the more people have to decide which subscription is essential to their life and work. Apps that could be replaced without significant loss of comfort and functionality or which could be replaced by a more versatile software package, will lose first.