I think that over-paying based upon what you hope a player will turn into is bad business. There are tons of players who never improved past their third season; season-to-season progression is a fallacy perpetuated by agents and bought into by fans, but it doesn't have a ton of basis in reality.
How many role players have suddenly developed into stars?
I agree in principle about paying for future projection; however, Middleton's *current* production is arguably worth 15-20% of the cap. He's averaged 17-5-3 as a starter with 40% 3PT and among the league leaders in advanced metrics. That's a very high threshold for "role player".
Also, it certainly doesn't happen every time, but many All-Stars make significant leaps 3+ years into their careers. Guys like Jimmy Butler, Klay Thompson, Chandler Parsons, Paul George, Derrick Favors, Gordon Hayward, Jeff Teague, Stephen Curry, even Rondo come to mind. Now many of them showed their ability early in their careers, but anyone locking them up at age 23 would have been handsomely rewarded.