All the examples I gave were of similar quality players to Modric in a central midfield position: "Felliani, Cabaye, Dembele or Sandro", I'm sure you only need to look at the transfer forum for several others abroad.
Again Modric is the type of player who has to be at the centre of everything to be effective, I just don't think he is good enough or mentally strong enough to perform that role for a big team, just as he rarely did it against big teams when at Spurs.
Welbeck is a young player who is improving. If he was 28 I'd say he was Spurs quality - which from now on will be referred to as just "good". A squad is generally filled with players who have the potential to be very good, players who are very good and players who are good but only fulfill a squad role (the likes of Fletcher, Park, O'Shea etc historically). Modric would not fall into any of those categories in my opinion.
When Modric was Anderson's age he'd just finished a decent but not great season at Spurs after several seasons in Croatia, whereas Anderson has played 158 times for Manchester United, earning a few improved contracts in the process. Also I think I've said twice Anderson only has the potential to be much better, not that he is.
Refer to the bolded.
Carrick and Modric would have nowhere near the legs to play as a midfield two together in my opinion. We'd also be heavily reliant on the wings as neither are adept at creating chances themselves (it'd be interesting to know their combined assists).
Either way here's 10 central midfielders I'd have way before Modric: Xavi, Iniesta, Alonso, Carrick, Toure, Schweinsteiger, Martínez, Pirlo, Ozil, Wilshere.