Based on past Mac platform transitions, I’d guess that Cletus’s take on it is probably closest to reality. “Just push a button and recompile” works great for Mac-only developers who write simpler applications that stick closely to Apple guidelines and make heavy use of Apple APIs. But Adobe is different.
Traditionally, Adobe has sort of operated its own “graphics OS” independent of Mac or Windows. Many Adobe applications don’t use the standard macOS text composition engine, window engine, graphics engine, etc. especially where Adobe thinks theirs works better, has more features, or works for them cross-platform. For example, for color management in Photoshop, the default system is the Adobe Color Engine, not Apple ColorSync.
In addition, Adobe applications use an increasing number of associated processes for all that cloud stuff, like licensing validation processes, sync processes, processes for cloud-supplied content integrated with applications such as tutorial videos and templates, maps, and so on.
All of that extra and proprietary stuff potentially creates a lot more “edge cases” than a Mac developer who uses straight Mac APIs (Core Graphics, Core Image, Core Text, etc.) for everything. All incompatible components, including those developed outside the control of the Lightroom team, must also be upgraded and made compatible, otherwise you get a situation like we have with Lightroom 6 where the app itself runs on Catalina, but the deal-breaking incompatibilities have to do with its mandatory components that come along for the ride, like the incompatible installer.
Historically, it means Adobe applications cannot be assumed to be a simple push-button recompile. But sometimes Adobe beats the competition, like when InDesign gained market share by transitioning from Classic Mac OS to Mac OS X to Intel long before QuarkXPress did. After that, though, XPress made the PowerPC-to-Intel CPU transition 7 months before InDesign did, according to Wikipedia.
For Lightroom Classic, which has 13 years of constantly changing Adobe requirements built into the code base, all of that means we users can’t really predict how quickly it will make it to ARM, and are going to have to wait and see.