You are arguing a point no one actually made. Perhaps, that's the problem.
Utilizing your goalkeeper in the build-up as another outfield player allows you to be more progressive with your attacking positioning. And most managers worth their salt nowadays opt for attacking structures that are rather adventurous. To put it simply, having the ball and taking the game to your opponent is the norm now in football.
Whether you want to play through the press or move the ball through the lines, it helps not to have players constantly dropping deep to receive the ball. Ederson doesn't assist goals for City, but he helps the team stay up the field. Alisson is an excellent link when Liverpool switch sides in their build-up quickly to move the ball forward.
ETH uses a rather aggressive 3-1-6 in the build-up. It usually consists of the two centre-halves and either FB. Don't you think that the team would benefit from having a GK able to participate in the build-up so that we can move both FBs or Casemiro further up the pitch? It's about adding options to what you're doing. It doesn't mean that De Gea is worthless and that he should be replaced by the next guy who's walking by OT.
I'll just remind you that Guardiola actually kicked Hart (City's captain) out of the training ground to get his preferred choice in. He also created one of the best sides in the world by showing faith to a GK who didn't excel at, pretty much, nothing else other than having good feet and leaving the sticks to sweep. Klopp spent a fortune (70 million Euros) on Alisson. I guess, it mattered to them.