Manchester United hold the record for highest fee paid for a player by any PL team in Pogba, the most expensive defender in the World in Maguire, have spent close to £1 billion in transfer fees in the last decade. (Unfortunately all of it in the wrong hands signing the likes of Sanchez, Di Maria, Schweinsteiger, Lukaku)
If Pochettino backs his ability to build a squad that wins the Premier League (or equivalent domestic league) and Champions League, and presumably he wants to do so, then joining United easily gives him the platform to build that team. The only other clubs that afford him that luxury are Madrid, Barcelona and Man City. At a push, Juventus.
Sure he can do that at Spurs will be your argument, but is a Tottenham playing Champions League football realistically attracting the same talent a Manchester United playing Champions League football? Consider how much more United can offer in wages and the size difference in the clubs.
You may also argue Manchester United are currently a worse team than Tottenham. Absolutely correct. Is that a big turn off for an incoming manager? Less pressure to deliver immediately and the bigger stamp you can put on the team over time in terms of your own players / playing style.
Tie in other factors, if United decide tomorrow “we want Pochettino, as our manager for the next 5 years at least and nobody else and we’ll go all out to get him” - United can offer him double his current wages easily, probably more, I’ve no doubt about that.
So yeah, If Pochettino is as ambitious as I think he is and believes in his own ability as much as I think he does, then I think he joins United. In fact if he didn’t (without another beast aka Madrid lined up) then I’d judge him and he automatically wouldn’t be the right man. And I don’t say this as a red, I say it as if I were in his position. Harry Kane for example would be a different case altogether, I’d strongly expect him to stay at Spurs if this 2019 version of United approached him and would advise him accordingly if I were his agent.