I think the reason lies in that Blind Guardian has never really played power metal, but speed metal and later speed metal with a folkish technique.
I'm not disagreeing with anything in your post but:
Power metal = speed metal after 1990.
That's as simple as it gets. Death metal technique, or Biohazard/Prong/Exhorder/Vio-Lence style extreme percussive metal, got worked into the mix, as did uptake of European speed/death (Kreator, Destruction) and melodic speed metal (Coroner), and attributes from some American bands like Slayer, Rigor Mortis, Devastation, etc. who were halfway between speed and death.
Modern power metal has a hammy feel to it because it's basically NWOBHM played by speed metal bands using death metal, black metal, grindcore, industrial and folk-rock technique.
I'll submit a humble but thought-provoking assertion:Blind Guardian is good because their composition is good.
If you played Blind Guardian on a kazoo with the Merry Pink Foaming Faggot Hamster Love Band playing accompaniment on an accordion, it would still be good, although harder to discern.
In the same way, Gorguts Obscura
is good because it is well-composed, as are parts of From Wisdom to Hate
, and Negativa is not bad because it's nu-metal-core with a stupid name, but because the composition is half-assed and trivial.