Seems a strange choice to pick SB for such an in-depth dive as this TBH? Stormblood set the tone even if they did have to walk it back a bit. ShB and EW followed the same trend.
HW seemingly ended on a very different development direction compared to how it started, I guess in reaction to how the average player took difficultly spikes seen in the likes of ThordanEX and Gordias.
As far as WHM itself goes. Yeah for sure, WHM is the best it's ever been in terms of relevance. I don't think anyone with any sense is going to deny that? I can think of 2 times in past expansions where WHM was a must pick for Savage (Gordias and Deltascape), on both occasions it wasn't because WHM was a powerhouse, but rather it was because one of the alternatives was just plain bad (AST for Gordias prog was a struggle right from A1S, SCH wasn't as bad but it still fell noticeably behind Noct AST in 4.0).
Even so, it's still not perfect. It's too mindless to play below Savage. For sure part of it is a content design problem, but job design has to take a part of the blame as well. Taking away healer utility from ARR and HW was an awful decision.
Just a small correction, WHM absolutely did not have MP issues in SB, SCH was the one having problems there surprisingly due to crazy inflated shield MP costs if I remember right? Thin Air did absolute wonders for the job right out of the gate and was arguably the most impactful ability of the expansion (It was the reason I didn't agree with the doom and gloom the forums had for WHM leading into 4.0). Remember that WHM was competitive for Deltascape, it just fell behind once SCH was fixed for the simple reason that it's healing throughput was GCD focused with no return like we have now meaning that it's DPS contribution fell off a cliff once it had to heal. If you dropped the current EW Lily/Misery system onto SB WHM, it would have been highly competitive.