I see, so you actually made it a tad easier at the lower levels, I missed that. Still don't entirely agree though.
I'm sorry to move the discussion off on a tangent, but if the role in war and adventuring is not also addressed, then a domain without access to individually powerful casters will always be at a disadvantage.
Right now, you hire in powerful people for casting, war and adventuring.
If we make casting abstracted, a domain thing, then you make it much more accessible over all. Many domains will be tempted to grab a few holdings of caster type, simply to gain the power therein.
Meaning, the boost to adventuring and war becomes much more targeted on individuals for this purpose alone. Rogues and Fighters and such would gain a popularity boost, perhaps at the cost of regents often being quite puny next to their hired help. - Their ECL is all that matters, blood and casting no longer does.
We see some trending of this, in Lannelah, the Green Knight and a few other super powered NPC's that can be played by domains crafty enough to turn them to their causes.
In most landed domains, losing the regent often means the rule falls to a younger descendant. (At least in optimal situations) - Temples and Wizard domains are at least more free here.
There is no reason to not have 2 level 11 priests, one the regent, the other the lieutenant and heir.
Same for Wizards, the Three Brother Mages would be an example I suppose, although they are running out of brothers fast.
I see this trend moving the regent back further to a position as figurehead and little else. No longer a Hero-King, just a King.
Apart from the setting, there is almost no connection back to D&D. I'm not even sure which side of the fence I'm on, against or for, I'm just pointing out that this change has radical impacts beyond realm magic alone.