The Raptors definitely benefitted from Siakam's development in 2018-19, but Kawhi was a one year rental for a mostly veteran team who had been good for many years but could never get over the hump. It was either continue being a 2nd round team until contracts ran out or go for the home run trade when it became available. That trade fit the timeline for that particular group.
If you add Durant, then yes you have him for 4 years (unless he demands a trade after 1), but he's also on the back end of his prime/career. So if Barnes doesn't really take off until year 3 or 4, then KD is kinda wasted here because you can't maximize his best remaining years. The only way for a KD trade to make sense is if they can keep Barnes and Siakam (to a lesser degree FVV as well but I'd be fine if he had to be moved). I'm also a little iffy on OG being able to take the next step. He's a very good two way 3+D player, but he can't create his own offense consistently and that's going to put a ceiling on him. The difference between him and Siakam is pretty significant, IMO. I'm more bullish on Achiuwa turning into something given his defense and improving ability to hit 3's. A center who can guard multiple positions and spread the floor on offense is a huge luxury.
A lineup of FVV/Barnes/KD/Siakam/Achiuwa is a title contender. Probably the best team in the East one through five. IMO either find a way to get KD with OG/Trent as the main pieces (with picks, etc, attached) or keep the status quo.