Salah isn't going anywhere and I don't think Liverpool are letting Mane go for anything less that £100m. You claim that United and Chelsea will rebuild and be challenging for the league again in a year or two. Haven't United been rebuilding for about 5 years now and gotten nowhere near? What makes you think that will change over the next 12-24 months?
The problem United are going to face this summer is the fact that Real, Bayern, Juve, and City are all going to be in the market for top quality players. All those clubs can at the very least match the wages United can put on the table but they can all offer a very high likelihood of trophies.
At this moment in time (not saying United aren't a great club) they aren't anywhere near those clubs in terms of attractiveness to top players. Why would any top professional worth their salt want to be getting dragged to the arse end of Ukraine on a Thursday when they could go elsewhere?
The manager at the moment is also an issue. I think any high level professional looking for a change is going to look at United and see a rookie manager who, despite being an incredible player, has achieved a couple of titles in a farmers league and relegated the first PL club he managed. Why choose Ole when you could go for Pep, Zidane, Klopp or Allegri?
Think about the team. If Pogba goes (which I see as likely) and there's a need for a new CM. Is a top level player going to look at the squad and think "Great, can't wait to get stuck in with McTominay and Lingard, brilliant" or are they going to look elsewhere to play with other top level talents?
United are, and probably always will be an elite club off the field, they aren't elite on it though. Once you've lost that elite status on the pitch it's so difficult to get it back. Look at Liverpool, they're still one of the biggest clubs on the planet but they've won feck all for the best part of 15 years now other than a couple of tin pots and no league for 30! Same with Arsenal, always there and there about but won jack shit and don't look remotely close to challenging.
I think there's a rude awakening coming for a lot of fans who perhaps don't see how far the rot goes at this club. If things start changing and the right decisions start being made immediately, then United can probably claw it back in 5 years. Otherwise, there's a lot more disappointment and heartbreak to come.