Going from Bayern to Tottenham is a step back rather than one forward. Yes he would be in the Premier League and it is all good, but there is nothing to be won with Tottenham now.
Going from Bayern to Tottenham is not a step back. He's not that big of coach to say he's too big to coach Tottenham. Coaches like Mourinho and Conte that have achieved a lot coached Tottenham so why is Nagelsmann too big for Tottenham? Bayern put a lot of faith in him when they hired him because he has never done a big team let alone be successful in one. Nagelsmann is a good coach but I cant say he's up there with the likes of Pep, Ancelotti, Mourinho. He will be a perfect fit for both Tottenham and Chelsea so if Tottenham actually want him they have to move things quickly and get him before the summer.
Going from Bayern to Tottenham is not a step back? Then tell me what it is please. I have no idea what you are talking about and how many titles Tottenham won in the past 20 years, but I do know that the club ranking tells a different story.
Apart from this list of course Tottenham is a step back. That doesn't mean Nagelsmann shouldn't go there, but he might have the chance to go to Real Madrid. I know what I would do if I were him. Why not go for the maximum? He coached Hoffenheim and then Leipzig already. He has his experience and knows how fast it can end.