I don't think Matic is a bad player, quite the contrary. The perception of him declining is flawed too, because a) he's not reached that age yet and b) players in his position and role don't need pace or even physicality at times. They can use game awareness to be of value to the team.
Having said that, I don't see the point of this signing except squad depth. The general consensus is it is to free Pogba, but do we need to include another body in defensive areas to do that?
The way I see it, the balance of this team is tilted towards the defensive side. We need either more bodies or more quality in attack. I don't see us getting a world-class attacker this summer, which means it has to be the former. So unless Pogba and Herrera are going to score 20 goals combined, having a midfield three of them plus Matic doesn't make sense to me.
As for Pogba, we could try giving him a free role in the 2-man midfield itself. Him and Kante worked pretty well in the France-England friendly and Herrera can replicate that partnership. Besides Pogba is not that bad defensively: he was great off the ball in the EL final.
I admit doing so will reduce our solidity in defense but isn't that what we need to do: tilt the balance more towards attack? As I said, Matic isn't a bad option to have, but I don't want him regularly starting.
Besides, he adds to our list of slow, sluggish players (Blind, Carrick, Fellaini, Rooney). Playing Matic will create problems similar to the ones for Ancelotti at Bayern, where Alonso started over Müller for 'balance'. As a result, Bayern dominated possession in big games without creating clear-cut chances from open play.