Bad scouting. Net spend is a red herring, as it fluctuates based upon a team's needs and wants per season. Wages is also a correlation to success, but not a cause of it. Remember, Spurs were 1 minute away from missing out on the CL final to a team that costs a fraction of the Spurs starting 11, brought together through good scouting and great youth development. Who is the better coach there, Ten Hag or Pochettino?
Games are won and lost by players, who are recruited and organised by managers, and trained by coaches. Having money only means you can aggregate the best talents to your team. Not having as much, doesn't mean you can't get top talents. Pochettino's ideal shape is a 1-4-2-3-1 playing a pressing game. Looking at his starting 11, to assess the quality of his team according to who he has available. He was bequeathed most of a good spine, that he didn't have to spend money on: Lloris, Vertonghen, Eriksen, Kane. That reduces his need to buy players for those positions. He brought in Alli, Son, Alderweireld, Moura, Sanchez, Sissoko, Wanyama, Llorente and Aurier. That's a strong first 11 and two off the bench, not to mention Trippier and Foyth. And he didn't have to sell any of his best players to do that. So I'm not really seeing this "miracle" work. Where is the "miracle", exactly? I just see very good management and what would be expected of a top 6 manager.
I kind of get the point you're making about players, but I stand by the point that you need a whole squad to compete in different competitions, because fatigue & injuries play a major part during a whole season, & the strongest squads survive through them. Others give up other targets & try to stick to one target to make it, and that target is relative to what is expected of them & the resources they have.
Okay, he had a good spine to start with, but many of these players have reached their peak under Pochettino than under anyone else (even Lloris has improved his consistency - he used to flap around more earlier). That is also a big thing to consider. Every player you have mentioned (except Llorente who is ageing and only a back-up & maybe Wanyama) have hit their peak and played their best football under Pochettino and no one else. What you're seeing now are developed versions of these players & then we say they're good, but there's also the thing. Has the coach made them better? In that sense, with & without resources, he is over-achieving.
And then there are clubs who get innumerable hits/attempts to improve their spine & squad, some succeed & Spurs are at a disadvantage to them & some still fail to do so. The fact that he has has a very little margin for error in transfers, as as it is he isn't getting much, then he has pretty much been spot on with most of his transfers. It's not just good scouting, it's plenty of team-work. It's how we also praise Klopp for the signings. Pochettino has even lesser power with Levy & still makes things work.
Talking about the budget of Ajax and Spurs, the fact is that Ajax are not competing in the Premier League. Make Ajax compete in the Premier League with their same budget, endure a full season of 4 competitions and then see if they'd have reached a CL Semi-Final with it. I think they'd be below Wolves frankly over the course of the season. That doesn't mean their CL run was weak, they were just able to save themselves, their energy & minds for the CL games, as they were comfortable in their league & domestic cup. But ask them to do it with a XI and without a squad in a league like PL, they'd not be able to produce consistent results at all. Wages/Net Spend is relative in your league & mostly doesn't apply to knock-out comps.
As for Ten Hag or Pochettino, if we're making judgments based on one-off games, then I mean, Hogdson has beaten Guardiola. Even if you are talking about reaching the CL final, they still didn't do it. Pochettino found weaknesses in Ajax's game, I'm sure he did this deliberately on purpose in both legs, they saved their energy for the 2nd half, tried to contain them in the first half and then went after them in the 2nd half. Ajax on the other hand, pressed like madmen and attacked hard in the first halves of both games, dropped off massively in the 2nd and left gaps in and around.
Ten Hag is a good coach, but ultimately the weaknesses in his side were brought out. This is his first season at this level, and Pochettino has been managing a Top 4 side in a Big 4 League, so it'll be a while before Ten Hag can be actually compared to him.
