Ya basically Avengers are dead without his funding and tech. Not to mention he was the strongest member of the G1 team and contributed the most against the Chitauri invasion. T'Challol ain't funding squat.
Since Spidey is canon, maybe Norman Osborne can get the reign started.
We have no idea what's going to happen with T'Challa, nor do we know where Tony's arc goes at the end of the movie. We also have no idea about how Infinity War is going to start, so I'm not sure where you're pulling this from. T'Challa could easily fill in, assuming the Avengers Initiative isn't completely disbanded. Assuming in the other direction that the Accords do pass, then the operation will probably be sold to/bought out by/assimilated into the government. It'd be interesting to see a Dark Reign scenario emerge from all this, I agree.
As for Tony being the strongest, Thor and Hulk are both far superior as far as pure strength and power go. Cap was established as a far better tactician and leader (as shown in the final battle), was the third strongest, and was most likely the best (maybe second best compared to Natasha) and he had a better understanding of what the Tesseract could do compared to the group initially. Widow was able to trick Loki into talking and managed to ultimately shut down the portal generator (even though Selvig was talking her through it). Banner's expertise in gamma tech and his raw power as Hulk speak for him automatically. Hell the only person who contributed the least to the effort against the invasion was Hawkeye, but that's only because he contributed the most to it and his arc for the remainder of the movie is a revenge/redemption arc (however small it may be).
Yes, Tony was a larger focal point -- though that's more due to popularity and being the "face" of the MCU at the time -- but to say he contributed the most and is the strongest is largely underselling the team. Tony had the tech and the brains (figuring out where Loki was, hacking into SHIELD, assisting Banner in tracking the Tesseract), but he was also the most major cause of friction because he insisted on doing things his way AND also getting used to power the portal (Stark Tower). Taking the nuke into space shutdown the Chitauri entirely, but there was no way to know that at all and even if the nuke did hit then the invasion would have stopped regardless, being that the portal would have been sealed and the army destroyed (though the collateral would have been considerably higher), so his only contribution there was reducing casualties (something that everyone was already doing by killing the Chitauri).
Come to think of it, I wonder if they'll just let it slide that the World Security Council was willing to destroy all of Manhattan, and that the Avengers actually reduced potential casualties even more. Or maybe they'll just blame it all on Hydra, which actually plays into the overall Hydra arc in a pretty interesting way.