Maybe so - maybe no. Unless Man City (in England) are coming in for you, then you're solely leaving for money; and those are the characters that Spurs are quite happy to let go of (I think Kane would be the exception to the rule). They buy younger talent up and blood them through the team so stop gaps are always filled. Likewise, they're still able to attract players from overseas by paying sufficient amounts - and maybe bonuses too, as mentioned by Stevo - so they lose Kyle Walker for £50m, but bring in Serge Aurier and Lucas Moura from PSG for that money. The stop gap (RWB) is filled and they've bought an additional player.
It'll be interesting to see if any club throws money at Spurs' players this summer. I actually think Alderweireld would be absolutely fine to leave - very replaceable, even if he's a good defender. Also noticed they bought Juan Foyth (for £10m) last summer as well; 20 year old Argentine centre back. So, again, buying young, blood them through. What would Alderweireld cost another club? £30m, say? They'll re-invest that amount with cash to spare, and the cycle will continue.
Again, as per original point, I don't understand people wanting this system to fail. It's madness.