Even the best of your mediocre supporting cast left.
Martin is way overpaid and shit the bed more often than not last season. Lamb and Jackson should be able to pick up the slack of his scoring.
You don't have a good supporting cast of role players. You have a terrible front court and you have no scoring off the bench outside I guess Reggie Jackson. If Lamb turns out ok your chances will improve though.
Jackson is a very good backup PG, Thabo is still an elite perimeter defender (and now he has a 3-point shot) and guys like Collison, Liggins, Ibaka and even Fisher (assuming we re-sign him) know their role and do it well. You're right that our playoff performance will likely come down to how well Ibaka and Lamb develop.
KD and YN will guarantee a good regular season but having Ibaka has your third best player is worrisome and your 4th best is who? OKC needs to make a move but I think their ownership is too cheap to take advantage of the talent they currently have.
Who can we add for the MLE that will really make us that much better? We can go over the cap signing someone like Korver, or get a decent amount of his production for a fraction of the cost for a guy like Dorell Wright. The returns start to diminish with guys like that, especially when there aren't many shots to go around as it is with guys like KD and YNB dominating possessions.
Presti was basically betting with the Harden trade that the pieces we'd keep instead of him (Ibaka, Perk, Lamb, Jackson, Adams) would contribute more around two already perimeter oriented players (KD and YNB) than Harden would on his own. Not saying I agree, but as talented as those three guys are it would leave a huge hole in the middle, and no money to fill it with.
The biggest X factor is still clearly Ibaka. I need to see him consistently posting 15 and 10 next season to feel OK about his contract and future ability to contribute.