What do DPS contribute to the party? Right, DPS. You know what Damage interacts with? Health. So all you need to do is to increase the health of the mobs proportionally and literally nothing would change - Except for a few select mechanics.
For your standard, run of the mill telegraphed AoE, it doesn't matter if there's one or one hundred DPS. It's when every player gets an individual marker or similar that things stop working, because you might run out of space to place the AoEs. I couldn't name an example in 4 man content off the bat, but that's where adjustments would actually need to be done. Though you could always just make the dungeons more difficult that way and tell people to deal - making them less faceroll in the process.
Tanking however doesn't get more difficult at all - the aggro of every DPS is tracked separately, otherwise tanks wouldn't work in 24 man raids at all. There are 15 DPS in those and tanks manage fine, because more DPS doesn't mean you need to generate more aggro.
Healing doesn't get considerably more difficult either - AoE heals have no fall-off, so group wide damage and single-target damage remain as is. If medica two can take care of the injuries of 2 DPS, it can also take care of the injuries of 200 equally injured DPS. If only one DPS gets targeted in the first place, there's no change either. The worst case of all DPS failing mechanics is the harsh part, because then you actually need to use more healing spells than you'd normally do.
It's not as much re-balancing as people make it out to be for 4-man groups. It's 8-man content and savage raids where the idea is more problematic - Alexander 12 Savage comes to mind, where you have your diffamation, tethers and the portals. A11 Savage as well, because there would be little space left at the clock positions. Those are some real issues that would need to be retroactively addressed on a case-by-case basis. 4-man content has not enough mechanics of that kind to matter IMO.