Let me clear something up. Currently of the players that we have aged less than 25 in our team, only John Stones and Luke Garbutt are players who have been signed by Moyes (and I'm being generous with Garbutt, he was basically brought through the academy).
However in contrast Martinez has brought in Robles, Besic, McCarthy, Lukaku and Galloway (albeit one for next year).
So for all that Martinez has brought in experienced players, he has also brought in fresh younger players forming a youthful core to replenish a clearly aging side.
And he's brought through and formed the team around Ross.
So I think he can quite clearly see that the team will drop off in the future and is bringing in younger faces to rebuild the squad as they say.