The information is out there regarding the structures. I'm not an expert just going by what was stated regarding the way that Nissan stadium was built very cheaply, mainly a concrete structure, and the renovation costs will be crazy. I don't know the itemization or specific timing outlay of the $1.8b estimated repair/reno cost but I do know the current contract leaves the city on the hook for that since whoever made the initial contract decided to reference that the city is responsible for providing/maintaining a "first class stadium" (paraphrase).
For the new stadium, the majority of the $2.1b is being paid by the Titans, NFL, and PSLs. Since I rarely go to Titans games it doesn't really matter to me. But I'll definitely be at every other major event they have at that new stadium if it interests me.
The thing is this isn't just about the old stadium or the new stadium. This offers up an opportunity to get an amazing new stadium and opens up all of that area close to the riverfront for additional development. I'll bet that area will be awesome once all of this is done and it will be one of the few stadium projects that has an overall positive impact on the city and state.