I would say Modric was excellent for us, not "alright". I assume Real Madrid agreed which is why they paid what they did. I don't think we appreciated how much Modric did for us, and we still haven't replaced him to this day.
A players' value is decided on what the buyer is willing to pay. It's as simple as that. Is Javi Martinez worth £34m? Not to me. But that's his 'value' because that is what Bayern needed and that's what they paid. Thiago Silva. Fernandinho. James Rodriguez. Lucas Moura. Illaramendi. Witsel. All huge sums, I would say at most 2 of those are world class, yet their fees suggest otherwise. These are not goal scorers, but the creative midfielders you speak of.
If no clubs needed Modric at the time, his value would be £0. His value only becomes necessary when another club wants his services. Then he is worth however many pennies we can squeeze out of them.
I wasn't saying he was only alright for us because back then we definitely more than an 'alright' team. I was just trying to point out he is a player who is capable of making the players around him play at their best.
Also the players you named don't fill the same role as Modric and were bought by teams which need to pay huge sums because they don't have the lure of Madrid so it's harder to force a move.