Ibaka had a really good western conference finals for the most part. I see nothing wrong with a PF that can knock down 3's, block shots, and has decent mobility. Sure, he could have better numbers on the boards, but part of that is due to his teammates being great rebounders. They were the best rebounding team in the league by far.
I really don't see Oladipo as a great fit next to Westbrook. Not sure whether he's a great defender, but I don't see him being a better fit defensively than Roberson is.
Like you said, they were a few good minutes away from the finals. And a big part of that was due to length and rebounding (in addition to the obvious of WB and Durant). They just traded away length at the PF position, and the main piece they get back in Oladipo will reduce their length (and rebounding?) at SG whenever they play him over Roberson. Or whatever you want to call it. If WB and Oladipo start next to each other you have two combo guards. Their whole collection of guards is pretty strange. Basically three combo guards who are all undersized to play the two and one legit 2 sized guy in Roberson who can't shoot well.
OKC just traded the 4th best player on a near finals team, and the main piece they got back was a combo guard that couldn't even consistently stay in the starting lineup on a lottery team last year. OKC did not receive the best player in the trade. Ilyasova is decent but he's definitely a downgrade from Ibaka because he doesn't have the defensive presence.
Whoever said this might have been better than getting Butler, no frigging way. Damn that team would have been scary if they could have just swapped Ibaka for Butler.