I wouldn't say players have regressed, they've just become disillusioned with Levy refusing to pay them what they would get at any other top club, whilst simultaneously blocking moves to said clubs with unrealistic transfer fees. From memory, Spurs had been in the Champions League once ever before Poch took the reigns (and that was almost entirely because they had Bale at his peak). Since Poch, they've been massively outspent by other clubs but have finished in the CL places every year, narrowly missed out on an EPL title and reached a Champions League final. The only reason people think he isn't as good as his reputation, is that he progressed Spurs so much that the expectations have changed.
They will never have a better chance of winning the title - one freak season all the big boys stuttered and they bottled it. I'd say that was a failure, not something to credit him with. And the Champions League final was a bit of a red herring - they stumbled into it with an enormous slice of luck (which everyone needs) and a favourable run, but then never turned up in the final. He also inherited a team that had Kane ready to step in the first team and become a brilliant striker. I get other teams have spent more, but under Poch they still spent over £250million in the last 5 years. Maybe he just didn't spend it that well?
Would he go to Arsenal if they offered him the job? Personally I think it would be great if he did, it would serve Levy right IMO
Tbf transfermarkt has it down as spent €390m but also received €284m. Tottenham had flirted with the top 4 before Poch arrived (reaching top 4 twice in the 5 seasons prior to his arrival), but Poch cemented their position in the top 4 and took them to a CL final. That's not a bad return for €106m outlay.
This is the question of 'What does success look like?' I would suggest that, for a club of Spurs' size, winning trophies should represent success rather than simply achieving a certain league position/making money. That's the mindset that Arsenal fell into under Arsene 'top 4 is a trophy' Wenger.
Apparently he is ready to come back - maybe fancies a short 6 month spell with us then ! Wouldn't have to move far..