I honestly don't know why so many people see problems with OHK spells which takes out an enemy but sees no problem with domination spells which not only takes out an enemy but also add the same enemy as an ally.
Have you considered that one of those things can be broken, interrupted, dispelled, or saved out of after the fact, while an actual old school save-or-die cannot be? Most players don't really think it's fair or fun to have their entire campaign ended by a single bad die roll, or having to reload their game multiple times because heir only option is to succeed a random chance roll, with no gradation or middle-ground. A DM who thinks it's fair or fun to levy save-or-dies agaisnt their party and risk complete and irretrievable character loss based on a single die roll, beginning form party level 7 or 8, just because, is also a pretty rotten DM.
A 5th level spell should not be able to one-shot any creature in existence for free. That's a ridiculous idea and a person who suggests that it should is being equally ridiculous.
Disintegrate is plenty powerful in 5e, compared to the setting and backdrop that it is situated in. In fact, it's overpowered for its level bracket in terms of its potential damage output and its character-loss consequences if it zeros someone. It is for that reason that it is one of the only high level spells in 5e that is still an all-or-nothing on the save, rather than a save-for-partial, like the vast majority of other mid to high level damage-dealing spells.
