What I mean is, lets say a raidwide hits for 50k, we have 60k max hp so we're not going to die to it. It doesn't matter if WHM can heal for 20k or 30k for example, as it's gonna take 2 GCDs either way. the extra 10k would not be useful. However, being able to reduce that 50k to 45k is, while 'surplus to requirements' (as it wont kill even if not mitigated), the point is that you CAN mitigate it. At no point is there a 'well your mit had literally zero effect' in the same way overhealing can have. As you say, it might be possible for natural regen to handle some things. But Mit can make things that 'cant' be handled by that natural regen, into 'can' situations. Doubly so when the Mit applies a regen effect of it's own.
Rather than having 'more burst healing tool' added each expansion for the Pure healers, more ways to restore smaller amounts, but more often, would probably work better. There's a reason I found Curtain Call so much easier on AST than WHM, and it wasn't just 'well they have like 4 regens'. It was that their healing tools (Star, CO, CU, Horoscope) are all 60s CDs. WHM just doesn't have any good 'short CD tools', cos it relies on the lilies for its design. Which is fine, but damn when you're out of lilies in highend content you're back to playing ARR.
Maybe a solution is to remove one half of what Barrier healers do, and make it Healer wide, either the shields or the Mit. Personally, I'd vote for Shields. This way, we could have eg WHM Temperance no longer applies 10% mit, but a fat regen like PVP. CU on AST no longer applies 10% mit, instead the regen is stronger while the barrier is channeled. For shielding, WHM can have Stoneskin as a Lily spender, and AST can have Noct/Diurnal midcombat swapping as part of it's gameplay. The Barrier healers will instead have ALL of the Mit, and also additionally room for 'mitigation via debuffing' such as Virus and Disable type skills. Some might say 'oh then the WHM will be bullied into using the shields, because it's damage neutral for them' and to that I say 'yeh, and its fine because it's damage neutral for them.' The WHM provides the shield and the fat HPS after the hit, the SCH provides the Soil to mitigate and Virus to debilitate.
Maybe not the perfect solution, but we seem to agree: Pures don't have enough 'stuff', Barriers have too much 'stuff', redistribution makes some sense