You... see the problem with that theory, right?
We don't have granularly-varied MP costs based on rank; our ranks are rarely even equivalents (Aero I, II, and III, Miasma I and II, Bio I and II, etc. each had significantly different profiles). We don't have the weirdly varied MP efficiencies. We don't have the separate CDs per rank. We don't have a significant Piety stat (or its equivalent in Spirit/MP5). And in dungeons, we didn't have MP concerns,
period, outside of the original Holy spam (for the highest burst damage in the game but nearly a fifth of max MP per pop).
(Granted, WoW dungeoneers generally also at least maintained DoTs; the "only heal" was only for in case of otherwise going oom in long fights. C'mon, in the less intensive dungeons, Druids would even cat-weave atop said DoTs, spending GCDs to change stance and fully DPS, in the less intensive dungeons in 2004. That's not an environment particularly conducive to princess healing, and it's only gotten less hospitable to that with time.)