We're passing judgment on him after 4 games?
The judgment is giving an extension to an unseen, declining player.
Is he really declining? His TS% last year was the best it had been in 3 years. TRB% and STL% best in 3 years. TOV% lower than the prior year. All of his WS numbers were better last year than the year before. His total number of shots has been declining (and he certainly had the ball in his hand less on teams with Simmons and Doncic than prior seasons), but he is scoring roughly the same amount of points per shot. You keep saying he is declining, but that isn't actually borne out of the numbers. He has been a very similar player over the last 3 seasons and given he is just 28, there is no reason to think he won't continue producing at a similar level.
He’s gone from a starter to a fringe rotation player. That matters. Good players tend to get more minutes, more shot attempts, etc.
16.6 ppg -> 13.7 -> 12.1 -> 6.3 ppg
3.2 FTAs -> 2.6 -> 1.8 -> 0.0
37.8% 3PT% -> 35.7% -> 34.1% -> 33.0%
And, his production is in decline, despite being matched up more against bench players.
It’s fair to wait and see. Extend and see is much riskier.
I'm not counting this year because he was extended before the season started, but here are some other numbers you didn't mention
2PT% - 45.8 -> 48.6 -> 50.1
FT% - 86.1 -> 80.9 -> 91.7
eFG% - 49.2 -> 49.6 -> 49.8
TS% - 53.6 -> 53.3 -> 53.7
RPG (TRB%) - 3.6 (5.5) -> 3.2 (5.7) -> 3.3 (6.0)
Doesn't look like a player that is declining at all. Looks like a player that has been moving around teams and having his role changed and adjusting to other teammates, but one that has performed pretty much the same over those 3 different seasons and 3 different teams i.e. Miami to Philly to Dallas.