In case of Diablo IV in my opinion Blizzard has a good track record of keeping game servers online for years and years.
That being said, the game does have some weird server hopping mechanic that you can't turn off, meaning it seems to switch servers while you're playing, which isn't always as seamless as you'd hope it would be. Also, at least for me, it sometimes selects servers with >100ms latency, which is quite noticeable of course.