I think it boils down to simply that. The last machine I dealt with that wasn’t implicitly a 64-bit system by default was the Pi, and even it’s (finally) looking to move to 64-bit. The limitation of not only many modern OSes moving to 64-bit only, but also the fact that the amount of RAM required for the OSes in general is creeping up, the physical limit of 4GB of RAM puts a very real damper on what KiCad would be able to do anyways.
Maintaining upstream packages that will absolutely drop 32-bit compatibility will only get more cumbersome as time goes on, and there are so many places better served by developer time than maintaining a legacy build type that is already only used by about 1 person in 20.
My opinion: With the current stable builds of KiCad being “feature complete”, I’d drop 32-bit as a target as soon as a needed upstream dependency required I do so. People stuck on 32-bit would be able to continue to use the last version available in the same way you’re still more than able to continue using Windows 2000 (the last exclusively 32-bit version of Windows!) – absolutely able, but quite unadvised.
That’s just my two bits!