I bought during the beta for a lifetime of goodies, had to buy it again after the buyout, then again after an update to MS accounts wasn’t acted on, and then for the Switch. I’ve bought Minecraft 4 times, with another on the horizon if it keeps popular.
Until it's possible to convert your world to Bedrock and not have anything in your 'finished' world break, except maybe some giant Redstone machine or one or two known annoyanced, I doubt they'd do it. Mojang presumably still has some autonomy within Microsoft so long as the money keeps coming in, and Mojang presumably knows that pushing this too early is a bad idea. But Microsoft being Microslop, who knows, maybe they'll just do it anyway.
So no. It is not one time purchase.
Then again, you'll never see a group of pre-schoolers wearing GTA5 hoodies and hats and backpacks, and you can't watch the GTA film in cinemas.
At least split screen still works for free.
I run a 6 person server on an Intel NUC, without major issue.