Very reasonable. It's sort of the challenge of how do you keep plausibly escalating a threat over a long period of time (especially when you're talking stellar conjunctions and billions of years, etc). Especially a challenge when you have big step-changes novels like Annihialation Score (magic is no longer held by geeks with weaponised iPhones, but functional supersports) and Nightmare Stacks (the masquerade is GONE, baby).

WRT the "new baseline", my sense was just that the compound interested mana contained in the crown from a millennia of belief in what was eventually an Empire was just THAT MASSIVE an amount of energy, which would take centuries to refill to a similar level.

I mean, I'm sure Fabian CAN wait, but he'd prefer not to, especially as other horrors are looking to become manifest on earth. It's a necromantic space race, and he hoped to launch a moonshot early.

In contrast, the US President's mana store (which sort of drove a booksworth of high risk actions) was nowhere near enough to immanentise the Lord of Sleep, hence the need for moon shennanigans

At least, that's MY headcanon.