I wouldn't say that they have neglected the product and I actually think that it's a ridiculous take. Until 2013 United were one of the top 5 teams in the world and the main asset was the best manager in football history, the particularity of United is that it was managed by the same man for 26 years and by the end he was essentially the club. The issue with the Glazers is that they didn't anticipate his departure, they didn't reduce his weight in the club which would have made his departure less crucial, objectively we can blame them for that because it's their responsibility but the truth is that most fans were against the idea of changing the structure of the club then and a lot are still against it now. I'm pretty sure that they didn't do it because SAF is Manchester United biggest icon.
Also people often talk about PSG and competing with them but PSG have been a mess since Leonardo left, they haven't really improved even though they spent a fortune and they haven't bridge the gap with the absolute best. The main issue for PSG is that they haven't replaced Leonardo, they lost his expertise and network, and no amount of money have made the difference because in sport the most important thing is expertise, money is great, it makes things easier but money can be thrown out of the window very easily, that's what PSG and United have done in the last 4 years.