In short, Day One Premium provides the steady, reliable income needed to maintain and grow the Day One platform. It pays for customer support, QA, maintenance, bug-fixes, and development of new features. Recurring revenue from this subscription ensures the Day One Team will continue to provide the level of quality and reliability you’ve come to expect from us… More at Day One.
$50/year sounds like a lot to me. I used to use Evernote for journaling and moved thousands of entries to Apple Notes quite easily.
If you have very specific needs, this may appeal, but for longevity alone I would prefer to use an app and service that is likely to be around for a very long time, and at a lower cost.