Old Trafford would definitely benefit from a partial upgrade, or even a cosmetic makeover - been a while since the aesthetically odd quadrant expansion. But spending loads of money on doing that, or expanding the stadium to 90,000+ doesn't make a lot of sense for the Glazers - who're businessmen first and foremost, and don't have a great vested interest in the support aspect of the club (like the Socios for Madrid, or the ~180,000 members of Barcelona). Matchday revenue has been trending downwards (as a part of the total revenue pie) for a while now with all the big commercial and broadcasting deals, so spending hundreds of millions on refurbishment (like the Santiago Bernabéu and Camp Nou projects) is unrealistic when the money is going to come out of their pockets and they aren't going to accrue significantly improved immediate returns (relative to commercial and broadcasting revenue):
http://www.bbc.com/news/business-37339740
Considering that, any major renovation project (or even replacement) is more likely to be undertaken when England holds a major international tournament - that's when these projects are subsidized and clubs frequently get advantageous deals. e.g. West Ham pay only £2.5m per year for the Olympic Stadium (which was built for more than half a billion), and Manchester City won the
stadium lottery.