Ferguson had a good few years to build a side, and then had the luxury of a pretty settled side for many years. This allowed him to only need one or two additions each season to add to what he already had, aswell as being able to max out on the kids coming through the academy. But even then he made some awful signings, spending stupid money on the likes of Veron, and also way over the odds on Anderson, Bebe, Kleberson and more.
Moyes took over a team that was coming to an end. Ferdinand, Giggs, van Persie, Carrick and others were coming to the ends of their careers and a major overhall was needed. But they spent only something like £70m in his time at the club when others were spending more. Had he been given time, I think Moyes could have got them back on track but after years of success everyone panicked and he got the boot. Since then van Gaal and Mourinho have come and gone and the club have wasted millions on players that are just not good enough.
When you look at their squad, who would be snapped up by the best teams in Europe? Pogba? Only at a massively reduced cost. de Gea? Yes, but even he looks like he's had enough. Rashford and Lingard could become the next Danny Welbecks/Theo Walcotts, Lukaku could end up back at somewhere like Everton!
The fact that they are miles behind Man City and Liverpool is almost acceptable, but to be miles behind the likes of Arsenal should be unforgivable.
