Here's a thought. Your iMac from 2015 employs an Intel processor. The writing has been on the wall for any Intel based Mac since the introduction of the M series. I was frankly amazed that they supported Monterey on as many models (yours included) as they did. I can foresee only the Mac Pro being supported in macOS 14, with support for Intel disappearing in macOS 15.

Edit: PowerPC support was droped after 3 releases - Tiger (10.4) being the first to support Intel, Lion (10.7) dropped support. Granted, that was over a 6 year period. 68K to PowerPC felt quicker...