I think the key mistake would be the "pandemic or no" part, not the "guaranteed billion" part. I mean, no, the Marvel name alone doesn't quite *guarantee* billion dollar revenue, but its worth a *lot*. Its just not enough to overcome major systemic headwind and a global entertainment market that hasn't even come close to stabilizing into its prior status quo.
Basically, figuring that the Marvel name value is enough to overcome all ordinary factors was probably fine. We just aren't dealing with anything one might have called "ordinary" factors ( and might not for anytime soon ).