I'm in the same situation with Aperture. It's not only a problem because of the issue you describe with losing the product of my prior work, but it's not clear what the migration target would be. The most obvious alternative is Lightroom, but this would mean trading a tool I have been using happily for free for 7 years for a 20EUR/month subscription service I would have to maintain the rest of my digital life.
My current solution is just to keep a bootable Mojave volume around, but long term I don't know.
Incidentally, I am thinking about writing an Aperture export/migration tool. I want to at least preserve the album structures, ordering within albums, version grouping/selection and image metadata.
I thought about doing this as a commercial project, but it's likely I'm the only one who cares (I have a large archive of historical photos organized using Aperture).
One thing I've learned: from now on, I will organize my image archives using my own software. I am done with trusting any large company.
You can always just pirate Lightroom (LR Classic, that is—the non-cloud-dependent one). It’s not particularly difficult, and sidesteps the subscription lock-in nicely.
My current solution is just to keep a bootable Mojave volume around, but long term I don't know.