We're not signing a player better than the ones we've let go, or the ones we have, simply because they're not getting a game unless Robbo breaks his leg.
So have faith in El Burro and our talented kids. Every elite club in the world has a similar drop off in key positions because they have the elite players in those positions and full backs for us are a key position.
This is such a hackneyed argument though.
Yes, there's a drop off between first choice and second choice for most clubs in the world, including elite teams.
But the drop off between Robbo/Trent and Milner is quite a bit more than say, Aguero and Jesus, or Suarez and Griezmann, or Muller and Coutinho, or Benzema and Jovic.
You're right that it would be difficult to get someone who would take many games off Robbo, but there are other options, like players that can fill in across a number of roles - like Milner but younger.
Someone like Zinchenko works wonders for City, for example.
As for the kids - we talked a lot about them and they've acquitted themselves well, but we're now thinking of loaning Brewster, and Larouci I think hasn't made it onto the bench for a PL/CL game yet this season? It doesn't exactly say "ready to take over from Robbo".
With that said, I don't think we're signing anyone in January unless major injury strikes, and we may have to get someone to replace Milner if he leaves without signing a new contract anyway, so I imagine this debate is moot.