Saying you need/should have 5 main healers in your guild because of that one fight is the same as saying you should have 3 main tanks for that one fight.
Then you come to Star Augur and you need to sit 66 % of your tanks!
Realistically, you mostly want 4 healers on the majority of fights, occasionally 3.
There is one fight in Nighthold where you just used an offspec 5th healer, just like you use a 3rd offspec tank every now and then.
Let's be real here, if you are in a guild that actually 2-healed progression, you wouldn't care about this issue. Might as well have one of your mandatory 5 geared up alts be a mainspec dps.
Having 4 main healers in a guild (with 1-2 decent offspec healers to fill in) is fine and it's really not that much worse than sitting as dps (sometimes you want to sit ~4 of your ~9 melee, then on Gul'dan you sit 4 of your 9 ranged).
There is a line and obviously a fight that required 8 tanks with tier set (that takes months to farm) to hit-cap taunts was not ok. It was doubly not fine when monks, DKs and DHs didn't exist, guardian spec didn't exist and paladins were alliance only and not really viable tanks anyway.
Current situation is decent enough. You can do heroic with 2 tanks and 1 healer per 5 players (say 4 healers and 14 dps).
For cutting edge mythic progression, there will always be some optimizations possible.
I don't think that these fights were designed with solo tanking or 3tanking in mind, it was the players that were thinking outside the boss and came out with offtanking drakes on the side on Cenarius or having a dps taunt Star Augur to reset debuff while the maintank is tanking the boss and handling the Gravitational Pull at the same time for the majority of the fight.
Just the same, 2 healing is never the intent in my opinion and when guilds found ways to utilize it (2 heal Star Augur to push last phase without killing an add), it was even hotfixed to discourage this practice.