Great this is JUST what we need... software developers are just going to LOVE this...
Ok so I can write code native for Intel CPU's.. OR... AMD CPU's, OR Apple CPU's, OR Google CPU's, or Snapdragon CPU's.. Or RISC CPU's (for DB vendors mostly)... and so on... but more...
This is like back in the day when you could buy a Mac with a Motorola chip, OR... a Power PC chip, and on Windows you could get Cyrix, Intel, or AMD with I think motorola having an entry or two there as well.
Except the M1, Power PC chip, the Motorola chip (68000), x86s, etc... are all different architectures.
But you simultaneously used an example of different manufacturers having software compatibility (Cyrix, Intel, AMD via x86) while also overlooking that different manufacturers had software compatibility... Qualcomm's Snapdragon, Apple's M1, Google's Whitechapel, Tegra, Mediatek, Allwinner, OMAP, etc... are all ARM based.
As in-house Apple's M1 CPU is, it's still entirely based on the ARM instruction set.
Compilers are getting pretty good at making universal binaries across groups of CPUs that adopt the same instruction sets. Source still will be available for Android.
Since Google releases source trees for every device (within reason) they've sold, kernel compatibility should essentially be the same as it is now, if not better. I'd actually expect software updates and support to improve once Google switches to their own SOC and other manufacturers potentially license it for their own devices. It's Qualcomm holding up updates due to proprietary blobs that don't get updated between kernel versions.
x86 is less encumbered by the hardware support situation as Qualcomm's SOCs are... and Chromebooks, regardless of who manufactures them, now get 8 years of updates from Google. All of Google's phones sold directly are bootloader unlockable. Chromebooks often use UBoot or CoreBoot, both open options.
I don't think there's much to worry about here. Ironically, the problem with fragmentation in Android is due to one single supplier's (Qualcomm) refusal to support hardware after 2 years.