You can easily solve latency spike issues by either having high enough bandwidth from your isp that you can service every device that demands bandwidth or you can get a router with qos. People in your house streaming shouldn’t be causing laggy games in 2022.
There are plenty of arpgs with solo play. Diablo needed to evolve if it wanted to capture a new audience as well as retain more players.
Im sorry, but no game needs to be online only to evolve.
Good example: Diablo 3. it's onlind only on pc, and yet, still works fine as an offline experience on consoles.
Let's be honest, blizzard's issues is with cheaters, so, why not make it so the offline characters can't join online?
It's a simple solution. hell, if you get the game on console and don't pay for a sub, you arent taking part on the online aspects anyway, besides being forced to be connected to the server for no reason at all.