Fenway Park, as it stands, is solid. I think they've done a good job with it. It's got 25-30 years of life left to it, at least. I don't think it's unsafe or threadbare or anything like that.
Wrigley Field, OTOH, is falling apart. If they can't renovate it - this
would be a good start - the Cubs should just tear it down and rebuild it from scratch, in the same location. The team can move to the Cell while this is going on (like the Yankees did at Shea in the mid 70's).