Simple: player loyalty. Some of the best managers (I'm thinking of Pep here, but there are others) create brilliant teams that win trophies galore, but at the end of the day, when they leave, the players themselves don't all yearn to go with them: they understand that they're playing for the club, not the coach, who just happened to be a very good one appointed by the club. Look at Dortmund post-Klopp, as one illustrative example: he took that club from mid-table obscurity to multiple Bundesliga titles over the course of seven years, but the players immediately knuckled down when Tuchel came in last June (seamlessly adopting Tuchel's somewhat different style), and none of them have made noises about wanting to follow Klopp to Liverpool despite his evident success as a manager and his development of those same players.
There is such a thing as players being too loyal to a coach over the club that pays their wages. It's one of the dangers that comes with appointing a charismatic manager like Poch who likes bringing through youth players and turning mediocre footballers into great ones: those players end up owing their careers to him in many cases, and it creates some tension when the manager (in this case, Poch) ups sticks to go to a club like United and then returns to tempt some of our players into joining him.
It's happened before, at S'oton: Rodriguez and Schneiderlin specifically wanted transfers to us because of their desire to play under Poch again to a considerable extent, judging by the news at the time (aside from our obvious appeal, of course), and there's obviously a danger of that happening here as well if Poch goes to a larger club that can then tempt our lads with higher wages and continued development under the coach that gave them their chance.
It's fine to be overly optimistic as a fan, but it's a terrible, terrible way to run a club. I do hope Levy's made plans for the possibility (and eventual certainty) of Poch leaving, and has tried his best to impress on our lads that, come what may, they're playing for Tottenham Hotspur, not Mauricio Pochettino. We've got a happy little situation here, with a good manager (and a good man, by all accounts) overseeing a good bunch of lads during an exciting time for the club, but that doesn't mean we can lose ourselves in the happiness of the moment and ignore the ramifications of it falling apart if that manager does leave.