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.