I know we are in a bad moment, but let's not pretend that playing for Manchester United is some sort of a sacrifice. You play for a great club in an iconic stadium, with a massive fanbase, and great wages.
Players play for money and success. I wonder how many actually end up playing for their childhood clubs. I'm sure there are plenty of players out there who grew up supporting United but the chance didn't present itself. But let's not pretend that players join Real Madrid just for the name, it's no coincidence that big clubs pay big wages.
So no, I don't think that they're mercenaries. They're professionals playing for a top club, and it's normal for a struggling big club to go after top players to get it back on top. Some players end up connecting with the club more than others, but you can't possibly label every players joining us on high wages a mercenary.