T/H/D/D vs T/3DPS - By the numbers

[Rilifane]

I'm kind of curious, can any Ultimtes be solo Tanked?

I've solo Healed Extremes without issue, but I've also solo Tanked extremes. And I think there was an all Tank run of something Savage (P1S?)

I hope this isn't you trying to steer this into a "all roles can be dropped" direction but yes, there is a 1/1/6 TEA clear. Solo tank AND solo heal.
Which, again, I don't think should be possible.

As for solo tank clears for endgame content, they are ironically the ones that are generally tied to more mindful and coordinated gameplay than solo healer clears (like the P1S solo tank clear having very specific requirements) while also being the ones with the most frequent guaranteed loss as opposed to rng loss at best like in TOP solo heal or simply no scripted loss, which is true for the majority of the fights.
Double tankbusters will often kill your top dps, there is no good rng to avoid it; you can sometimes avoid it by coordinated mitigation stacking and making sure the most tanky dps takes the hit but good luck trying that with busters like in e8s. So for many fights you would need to constantly sac your top dps in order to pull it off (almost all savage fights, Dia Ex, Barbie Ex etc) - the execution is inherently "not clean".

Another difference is how much of each role's toolkit is dedicated to which task.
DPS have almost all of their toolkit directly dedicated to dealing damage which, barring a few examples in Ultimates when you tempoarily need controlled dps, is always useful by increasing efficiency and lowering the risks by potentially skipping mechanics.
Bloodbath, Second Wind, Addle/ Feint, Samba etc which is is 5 buttons at most with most having 3.
Tanks have a more even split with about half the toolkit for mitigation and aggro and half for dps, give or take a few % depending on the class.
Now healers are the other extreme with everything BUT 4 to 6 buttons dedicated to dps and the rest to healing.

I'm mentioning this because healing is completely binary. Aggro is binary, mitigation has diminished usefulness. Dps is always useful.
So even when you are not strictly required on DPS because the content you are doing has no or a very lenient dps check, your entire toolkit is still designed for something that is always useful, that contributes to a faster kill or potentially skipping later and more dangerous mechanics. You are always a gain.
Once the binary healing requirement has been fulfilled however, a healer is a gimped dps. And if the binary healing requirement can be reliably fulfilled despite dropping one or even both healers then you really are just a hindrance because you're lowering the speed or the chance of skipping mechanics for no good reason. And if this is possible in endgame content, then we have a problem.

So being able to drop tanks is just as bad as being able to drop healers and being able to drop DPS and intentionally gimping the party isn't an argument that it's okay either.