If you go to other stadiums semi-regularly it becomes fairly apparent how dated OT is. I hang around with a group of mixed football fans and on a basic level they don't like going to Old Trafford because it's uncomfortable and the facilities are generally worse. Which sucks for me because I end up going to away games more but you can't really argue with their logic.
The reality is that a lot of the people who attend games aren't hardcore football/United fans who only care about things that directly impact their view of the game and are happy to dismiss other aspects as "luxuries". There are also families, kids, casual football fans and people working at OT to consider too. And the "luxuries" of modern stadiums matter to them, on top of the more pressing issues like the roof or whatever. We're supposed to be the best club in England, so why shouldn't we offer the best experience to people turning up to watch us play, for whatever reason?
Case in point: one thing I have heard journalists complaining about for years is the WIFI at OT. Now while that doesn't impact on someone who just wants to watch the football, it does impact on the media working from OT every week. And given how basic a requirement wifi is for them, it's pretty ridiculous that a club of our wealth wasn't able to provide it to a decent standard. Specifically I remember Daniel Taylor pointing out that you could get better signal in the Masai Mara than in OT, which is just stupid.
You could also point to something like the long-standing (though hopefully now addressed) issues Old Trafford had with rodents as evidence of the club's lack of care for the stadium. It's not good that we reached the point where the council were publicly criticising us for not dealing with it appropriately in 2015.
I also can't imagine how difficult it would be for anyone with mobility issues to navigate OT either. Which, again, should be a pretty basic requirement in a modern stadium.
When you look at our peers in Europe, Bayern, Juve, Atletico, Arsenal and City have all moved into new stadiums during the Glazers' time here. The San Siro is currently due for a rebuild. Real Madrid and Barcelona are doing major rennovations on their stadiums. I think Dortmund did some renovation during that period too? Meanwhile Old Trafford has just been left looking shabbier with every passing year. It all speaks to the general lack of care from the owners for the club.
We talk about Old Trafford having prestige, history and soul but that doesn't ameliorate a lack of care and modernisation. In fact it's a reason for there to be all the more care and modernisation. It's something that needs to be maintained through renovation, not left to decline in the past.