EDIT: I'm getting downvoted, maybe because I was too cryptic. To clarify, it seems like the rumors that gain traction with mainstream sites and Apple-focused YouTube channels and forums tend to have strong correlation with something that will happen later.
They're not going to remove the ability to run arbitrary code or the unix core. There would literally be no reason to buy a mac over another product of theirs.
Yes, they won't support dual booting linux - you can run a VM, but if that's a dealbreaker - fair enough.
There's absolutely no chance however that savvy users will not be able to continue running non app store apps.
Apple want control, but they're not stupid enough to completely lock down the development machines for their entire ecosystem.
There isn't much people can do about that though? You basically have to run their OS to develop apps for Apple products, and people who don't develop apps for Apple products don't use Mac's anyway. Just make a laptop appstore with everything you need to develop Apple products and they could force all programs to go on it and they would lose almost no users of their laptops.