Apple won't drop support for Intel apps next year. Even if we assume that macOS 15 drops support for Intel machines (and I still think there's a chance the 2020 Intel machines might get another year), there's no way they're dropping Rosetta 2 at the same time.Considering Apple will likely drop support for Intel next year, that makes no sense.
I expect they'll do the same thing they did when dropping 32-bit Intel apps: they'll announce deprecation and then have the Finder start giving a "this application won't work with future versions of macOS" to try and shame the remaining stragglers. Although... hmm... did they do that with PPC apps? I have a machine with 10.6.8 so I could try and check, but I'm not sure if they gave users any warning that 10.7 would drop PPC apps.
I actually wish they would start giving those kinds of dialog boxes now because, well, that kind of shaming is the only thing that will get the remaining developers moving over to AS-native apps. Everybody else, whether it's the Big Developers (Adobe, Microsoft), the Mac loyalists (PCalc, BBedit, GraphicConverter, etc. that have been around since 68K), Big Open Source (e.g. LibreOffice), or the Windows-first reasonable non-Electron folks (e.g. Vandyke's SecureCRT, UltraEdit, etc - the people who all started porting their Windows software to the Mac in the Intel era and who continue to operate on Windows timelines), they're all AS-native by now.