'A player, especially a young one, who can't adapt and play in more that one position, isn't actually a good player at all.'
Absolutely agree.
But our Moussa Dembele stared professional life as a striker but was moved back to CM in his early 20's very simply because he wasn't a very good striker (a low scoring record is a good clue) whereas moved back he was a far better CM.
So when Harry is out Moussa is an alternative - but I'd guess Poch hasn't tried him there as there are far better alternatives as forwards than Moussa.
Likewise Luke Amos started aged 16 as being an AM but was moved back and is a decent DM/CM, but moved to LB he's no doubt developed his general game but after 6 months playing there, its apparent that's not his best position. If he had then played as a CM/DM for the next year his abilities in that role is likely to have improved him - but instead he was played as a LB for an extra year as there were no decent alternatives, and I doubt he improved his skills to be a CM/DM.
Danny Rose was a LW when he arrived from Leeds but in Spurs academy he played for at least a season as a CM, and it was only when out on loan at Bristol City he was played as a LB, and it was found he was decent there.
Poch has brought him on as a CM to see out a game, but never (that I can recall) started him as a CM, only as a LB. Reason is that he is not exceptional as a CM, but he was (and might be again) a very decent LB or LEB.
Harry Kane started as a goalkeeper but by his early teens was played as a striker. In an England youth match (u18's ?) he was brought off the bench at HT to replace a CM and everyone thought the coach was nuts as there were other very good cm's on the bench. The joke was on us as Kane was the best midfielder on the pitch in the 2nd half showing fantastic passing from a Carrick like position. We know he's a very good striker, but if Poch needed him to drop back to CM, I'm sure he could do a job - although he tried a few minutes of being goalkeeper in the first team....and not to be repeated I think.
So my argument is not that players can only play in one position., but that most players find that they are far better in only 1/2/3/5 positions and not so good in others. So at some stage you specialise in those roles and practice how to get better in those roles, and if it doesn't work out the coach might try you in another role later in your career (like Moussa and Vertonghen) and if it works you then need time playing in the new role to get as good as possible in that role..