It's the last season on the cheap for Simmons, Kyle has 2 more at the rookie level and they really need him. Simmons, if Dejounte and Bertans come out balling and become better perimeter players than him through the course of the season, I could also see him lose playing time and being considered for a trade but even then, the Spurs likely hold on to him this season for depth.
And to your point... at some point Spurs will have to start paying their young players $, instead of giving lifetime achievement awards to Manu and Parker. It will be easier to retain Simmons if he turns into a difference maker than luring a difference maker here in the open market bottom line, and if he doesn't turn into that difference maker then farewell to him you let him go.
Also, Crabbe looked like his first two years in the league and it wasn't until his third season when playing time opened up for him that he improved significantly.
Ultimately, I don't have a hot take. Kyle is due for a breakout season and is 22, younger than Crabbe in his own breakout season, young enough to have been a rook coming into this season. Simmons is pretty much what he is. You are unlikely to find a point forward who can shoot the 3 for cheap though and with Manu likely retiring next season, Anderson is a must keep.