I think that is all very fair speculation and analysis. I don't think there's a hard ceiling, but as a general rule, this seems like it might be as far as he can take a team. He can mold them into a good, functioning team, even out of bad circumstances, but can't get them over the hump to elite. Which would make sense when the best part of what he brings is order, organization, stability, and consistency, but not necessarily innovation or insight.
Which is why it would be so important that, if you value his stability as Jerry clearly does, that you shake things up somewhere else. Like OC and QB coach.