I don't see the disparity between Dembele and Mason. Mason has been ahead of Dembele in the pecking order for about 18 of the last 24 months, and has been injured most of the rest of the time. And Bentaleb as 4th CM is as anyone we could bring in for £20m+.
We've already got 8 senior AMs for 3 spots. Even if Chadli moves on, there's the likes of Onomah, Njie, Pritchard, and Oduwa already poised on the production line.
If you do not see the disparity in ability between Ryan Mason and Mousa Dembele then that is all anyone needs to know about your knowledge or therefore lack of relative to the conversation. Making statements often enough does not make them fact, no matter how many times you repeat it or ignore attempts from others to kill your outlandish claims.
Mason is not technically in competition with Dembele anyway as Mason's position throughout his time coming through at Spurs is as a number 10 where Dembele has played a few times only this season. Mason is nowhere near comparable as a DM with Dembele, its not even worth trying to discuss why that is the case, it just is.
Bentaleb may well be just as good technically as anyone we could bring in for £20m + but that is completely worthless if he doesn't buy into the philosophy of the manager, is injured all season and then decides that playing a friendly for Algeria is more important than managing his return from injury in a manner that the club deems responsible. May as well sell him and get the £20m and bring one of the youngsters through "off the production line" that you speak of.
These AMs that are being discussed, Oduwa is still a way from being able to be part of the first team squad on a regular basis. Pritchard has lost an entire season and will probably have to do a 6 month loan where he actually plays before coming back into the squad, Onomah is not really the sort of player that is required in the AM area imo, certainly not as a wide forward. That leaves N'jie who can play as a forward or AM, Chadli, Eriksen, Lamela, Alli and Son who will miss much of pre season and the start of the new season by playing for South Korea in the Olympics.
Signing another wide forward given the increased number of games we will hopefully play with a slightly bigger squad is not a bad thing to do, especially if a player of Schurrle's quality is available. Do we just pass up on buying proven quality because you feel 3 signings is now too much disruption for this squad?