Because... I've seen them?
Sterling is one of the best players the club has ever had, five assists away from being the only player to get 100+ goals and 100+ assists in a City shirt. Anyone who thinks Edozie or Borges can fill his slot without a massive drop in quality is deluding themselves. That's no knock on them. I like them. They're not Sterling-level. And out of all the names you list, only Foden is the dynamic and rapid winger and even he prefers being inside rather than working the sides of the pitch. Grealish is that even more so, Mahrez prefers cutting in, Haaland and Alvarez play in the middle, the rest are academy players, and none of them have the pace/width/power that Sterling brings. That means we'll have gone from three fast, wide, dynamic wingers who enjoy working the sides to none. Obviously we will still have fantastic players but it's a dimension to our game that will have been depleted heavily. How much of an effect that will have is a mystery, but I'd rather just not have to worry about it. Keeping Sterling should the priority if he can be convinced to give playing with Haaland a go, and if he does leave, a player with a similar profile should be the priority over more inside, deep-lying, tricky players like Felix. Just my tuppence.