He has no stamina issues and is not injury prone. There is zero reason to replace him with a pub player like Gagliardini unless Conte feels Eriksen is not performing at the requisite level.
Eh? It's because Gagliardini is
inherently more defensive as a player.
I think there's such a fallacy to subbing out a possession player like Sensi or Eriksen for a defensive player like Gagliardini. When you take out a ball playing mid, you lose exactly that - the ability to play the ball in midfield.
When Sensi and/or Eriksen are on the pitch, they play a significant role in helping us keep possession. They are comfortable receiving the ball in tight spaces and recycling possession calmly and efficiently. Sensi in particular is partial to winning fouls and making nice, short dribbles here and there while Eriksen can launch counter attacks with quick, accurate balls to Lukaku. Neither is a slouch defensively either.
On that note, Gagliardini, while more defensive, is not exactly a defensive specialist himself - he's simply more defensive by definition because he's not very capable in offense.
Bringing on Gagliardini for Eriksen does not lock down the midfield like it would bringing on a prime Cambiasso or Kante. Instead, the irony is that we often become defensively
weaker because we lose a key outlet to retaining possession. It also puts far greater pressure on Brozovic to do all of this himself and we know he can make mistakes when pressured heavily - both by the opponents, and the tactical responsibilities placed upon him.
I'm not going to resort to old cliche's like "offense is the best form of defense" but there's an element of truth to that. When you withdraw or reduce your attacking capabilities, the opponent knows they can attack you with impunity and without fear of retribution. How many times have we suffered through this late in games?
Who am I to question Conte. All I see with my eyes is that when Eriksen comes off for Gagliardini, we lose a ton of fluidity in midfield and we struggle to play the ball out from deep. We lose an accurate passer who can create that offensive transition. Instead, we repeatedly waste possession by booting shit-tier balls to Lukaku and invite the opponents to attack us over and over again. On top of all that, I don't feel Gagliardini adds a notable difference defensively either, at least not regularly enough.