This has been a recurring conversation for as long as Heavensward has been out and the best balanced solution would indeed be to give MND bonus for one healer and PIE bonus for 2 healers, regardless of party composition. It's simple and elegant.

They don't even need to get rid of PIE this way, no harm done to BLMs, best of both worlds, future-proof etc.

Unless career WHMs see the removal of an exclusive MND bonus as a nerf that would threaten their party slot (?) I see no reason to not update this.