Phasing out Rosetta 2 seems like a reasonable move. Maintaining backward compatibility indefinitely adds complexity and technical debt. Apple has supported Intel-based systems for a long time, and this step aligns with their goal of keeping macOS streamlined for Apple Silicon.
I wouldn't call 6 years a long time for support. Imagine if Microsoft announced that any software older than 2020 will not longer work. Not be out of support or not get any more updates, just become not runnable.
The problem I have with it is Apple unilaterally deciding that support ends. I don't see the harm in no longer supporting it but leaving it as an option for legacy support. No garentee that anything will work with it and no support for it. They've done this with their hardware before but here it's just a cudgel to force devs to update their apps.
Backwards compatibility may be many things, but it is never technical debt.