TBN was added in Stormblood as an answer to Sheltron working on all damage types. In exchange, Intervention was added so that you could apply a similar effect to your party members. Raw Intuition was repurposed in Shadowbringers to give WAR an equivalent, and GNB gained Heart of Stone on release.

Sheltron, Raw Intuition, and HoS were all upgraded this expansion to add additional defensive effects and self-sustain. TBN didn't because it relies on the shield breaking to remain damage neutral, so Oblation was added as a separate action rather than going the same route as the others. If the MP cost didn't exist, it would have likely ended up designed similar to Arcane Crest, where an upgraded TBN/Oblation breaks to give you a heal/regen. I suspect that's what they did originally aiming for before realizing that it wasn't going to work in practice. Oblation has a very impressive animation for a thoroughly underwhelming effect.

I think adding built in heals to Holy Spirit/Circle was a mistake. I would have much rather that they had given PLD some sort of gameplay condition to gain a swiftcast proc for Clemency to make it situationally damage neutral, while leaving in the option to hardcast it in emergencies.

I also wish that a Shield Swipe approach could have been incorporated into Bash to preserve that 'counter' style gameplay. Bash definitely has niche uses in certain content types, but it's a fairly boring action.