They're not going to pack their bags and say goodbye - you'll just stop getting the exciting new features; or will get them long after everyone else does.
MG Siegler did a good job of explaining your future here:
I hope they like current tech because they may not get future tech...
spyglass.org
Ben Thompson did an even better job, but it's a much longer article
Recent E.U. regulatory decisions cross the line from market correction to property theft; if the E.U. continues down this path they are likely to see fewer new features and no new companies.
stratechery.com
For what it's worth, both authors agree Apple should open up the App Store and allow side loading, they just think the DMA is a terrible way to do it.