If the roof weren't historic, wouldn't the entire history be different? I mean, in that scenario, SCC maybe doesn't consider the building such a city icon, and a different use of the space in a largely residential neighborhood is found 8 years ago. And, then, Hansen's arena in SoDo happens instead because SCC doesn't feel the need to protect Key.
But, yes, it is very true that the cost of the remodel is so high because of keeping the roof intact.
As for ROI.....We'll have to wait and see, because the sponsorships you discuss here go, in other markets, to the teams because game night revenues are not enough to keep the solvent. I know it is in many ways the same group running the arena and owning the team(s), so it's like one big pot in a way. As for the arena and concerts, etc....We know that some other civic arenas actually run at a loss. So, 20M a year profit (not including interest and inflation) seems like a big ask.
But again, it's going to happen. And, the team(s), once there, are going to stay. There is no question on that. The speculation is all on the arena side. And, that's private money, so they can invest it however they want.