IMO this sort of spending should be almost, if not entirely, outside the realm of FFP. There probably needs to be some equity in what teams are allowed to spend on their actual squads, but if a billionaire owner wants to come in and invest in infrastructure for the club they shouldn’t be hamstrung in their efforts to do so. To massively oversimplify the situation, that’s how Everton ended up getting deducted points last year, because of cost overruns on the stadium. In fairness, it’s not as though we didn’t also spend recklessly over the years.
As you said, the investments City have made over the past 15 years have been nothing but a positive for that community. It’s no wonder United are wanting to upgrade/replace Old Trafford. It’s kind of a holdover from the ****hole era of Manchester you described. Area around it is decent enough, but OT itself sucks. I’m hoping Everton’s new ground will have a similar effect on a smaller scale for the derelict, abandoned area of the Liverpool waterfront that they’ve built it on. Also, unless you’re West Ham and just happen to fall into a free one, stadiums in the UK aren’t publicly funded. That leaves the ownership having to invest their own money if they ever want to improve.