Apple started making this transition in OS X 10.5, which came out in late 2007. The 32-bit-only Carbon APIs have been deprecated since 10.8 (2012). It's not like this snuck up on developers; the writing was on the wall by the end of the last decade.
I'm not unsympathetic -- I have some 32-bit programs lurking around, including Dramatica Story Expert, an expensive one that's still theoretically being updated. But if that program stops working, I'm inclined to blame the developers, not Apple. Apple isn't forcing them to put out an app that feels like something from 20 years ago, or to consistently wait until beyond the last minute to update for system transitions that they've had literally years of warning about. (In Dramatica's case, they didn't even transition to Carbon until the classic environment went away, and didn't transition to Intel code instead of PowerPC code until Rosetta went away.)
Nope, I am afraid that won't protect you from the curse of abandonware. Only an emulator can. Your problem is not related to what type of hardware, software, OS you use but how old they are. Nothing is supported forever. With emulators we keep using technology that is more than 30 years old. This is a walk in the park.
This, or go with idea that for some years ahead you can work with El Capitan on a old hardware. Windows for me is dead end also, until someone in pro industry realizes that its time to move to Linux. Apple is mobile and entertainment company now. Microsoft wants AI/Cloud. The logical answer is Linux.
>until someone in pro industry realizes that its time to move to Linux
Some already are. When I bought my copy of Pianoteq 6 [1], I was surprised to see they also offer a Linux version (and even a Raspberry Pi version). The DAW I use called Renoise [2] has a Linux version, and the JUCE [3] development framework common to many VST plugins also has Linux support.
Listening to podcasts like Sonic Talk [4], I'm mostly hearing about studios switching from Mac back to Windows, but they have been talking a lot more about Linux too.
I'm not unsympathetic -- I have some 32-bit programs lurking around, including Dramatica Story Expert, an expensive one that's still theoretically being updated. But if that program stops working, I'm inclined to blame the developers, not Apple. Apple isn't forcing them to put out an app that feels like something from 20 years ago, or to consistently wait until beyond the last minute to update for system transitions that they've had literally years of warning about. (In Dramatica's case, they didn't even transition to Carbon until the classic environment went away, and didn't transition to Intel code instead of PowerPC code until Rosetta went away.)