Calibre has always been a small price to pay, but if sideloading goes away, I’ll certainly never “upgrade” again, and I’ll trash my 11th gen Paperwhite if they somehow make it stop working. Usable e-ink ereaders are even doable as DIY projects now, and Kobo will probably stay less closed-off than Amazon for a good while.
That said, reading the comments and the article it seems like as long as your OS (or some app) supports MTP, everything should still work more or less as it has, which is to say kind of annoying and with Amazon pulling little microaggressions like deleting your cover thumbnails, but overall sideloading should still function.
Seems like they don’t work exactly the same as they used to, as they now use MTP instead of USB mass storage, but while annoying, it’s generally a pretty trivial fix and your OS may already use MTP devices with no trouble. It seems there may be some other knock-on effects with fonts not sideloading right and needing a Calibre plugin to make pagination work how it used to.
So yeah, it’s getting worse, but Amazon hasn’t figured out how to bring the hammer down yet.