I'm not surprised by the Okada win. He doesn't need to be champ forever, but the thing is, he's the guy you're investing in. I don't follow puroresu as much as I should, but they seem to have certain spots "locked." Okada is now in the Tanahashi spot, whereas Tanahashi is now Tanahashi. Okada made history in 2017 by becoming he longest reigning IWGP Champ ever. There were probably people tuning in for the first time since that happened, and they probably wanted to drill home what a living legend Okada is, so they can bank on him even further for the next five years.
Tanahashi retaining was also a prediction I switched to. He's hurt, but to have him drop the IC Title and then disappear is a big fall for the nation's hero, you know? Tanahashi disappearing with the IC Title assures people that he will be back. Not sure when their New Beginning show airs, but it wouldn't surprise me if there is some sort of angle where someone (Suzuki wouldn't surprise me, but my money is on Naito for some reason) takes out Tanahashi to give him a Wrestle Kingdom 13 opponent. I kind of expect Tanahashi to hibernate until the G1, then he returns, doesn't quite get the match he wants against that guy, and then he goes on to face them for the IC Title at WK13. Tanahashi vs. Naito would be a cracking match. But so would Tanahashi vs. Suzuki or even Tanahashi vs. Jericho, should he decide he wants to be a heel in Japan again.
I think Okada is dropping the IWGP Title sometime around Dominion or mid-year (not sure what their major show is around that time). Ishii may not have been the wisest choice, but I can see someone taking it and then Okada winning it back at Wrestle Kingdom. He will be in that main event, the question is just against who. Omega seems...done and locked up. Suzuki or even Naito, given their history of running main events between "their guys" makes sense, but for some reason I just can't see Suzuki headlining.
Of course, maybe Brock Lesnar jumps ship and beats Okada for the belt...just saying.