The era of 32 bit Mac applications is probably coming to an end. Apple seems to be moving towards a 64 bit future.
64 bit conversion isn’t very difficult for most ordinary apps, although there will undoubtably be developers who will face severe challenges due to particular circumstances. Apps that include 3rd party code, complicated build systems or involving languages other than C/C++/Objective C are probably at some risk.
But I think most currently sold applications are already 64 bit and have been for years. (SQLEditor went to 64 bit only a while back, without anyone making any comment whatsoever).
The biggest loss though is probably going to be 32 bit only games. Games don’t normally get much in the way of updates anyway, and the likelihood of a new 64 bit conversion is low.
We saw this happen when iOS went to 64bit in iOS 11. Some developers simply couldn’t rewrite their games for 64 bit, for various understandable reasons, so the games are simply going to be removed from sale. One comment was that the particular version of the 3rd party game engine they used did not support 64 bit. To update would require a significant rewrite, not merely a recompilation.
The same also happened with the move from PowerPC to Intel. There are quite a lot of old games that were produced for PowerPC, that were never converted to Intel. Also true  to a lesser extent with the 68k to PowerPC conversion and the Classic to OS X conversion.
Games have essentially been fixed artefacts, they existed in the moment and remained as they were originally sold. Far more so than many applications, where the first release is often merely a promise of some future, better, version. Admittedly games are moving to the regular updates model as well. Often with excellent results like Blizzard with Starcraft, or Introvision’s Prison Architect, both of which have received regular updates and big improvements since launch.
I think the outcome will be that there will be quite a lot of 32 bit Mac games that won’t be playable in the future, and which will never receive updates, which makes me sad. With luck, new games will be released that I can enjoy too, but the loss of old favourites will be a disappointment.
There is one potential silver lining, the desire for all things retro and for remastered editions of old favourites on App stores. The fact that the original will no longer be available for sale may increase the market for a remastered version for newer platforms.
I can but hope 🙂