Not sure about Platinum after Scalebound but time heals all wounds. Obsidian would not have wanted to be acquired otherwise.
Obsidian's scenario showed that everything is possible. They worked with Microsoft on new original IP which was cancelled. Also, Obsidian openly said they were open to be acquired (September 2017).
I don't know how PlatinumGames situation looks like now. It could be great addition to the Microsoft Studios' family. However, we don't realy know if they want to be sold-off. Also,
Scalebound is only one game in their history, if checked this good, which was cancelled. Of course, delaying even until 2019/2020 would be much better than this. But… It happened. There is no point to go back again with that story.
Another thing is more important than just buy something. Microsoft and their managers have to really learn and understand Japanese approach to the business and culture in this country. It's quite different. This is very modern country which is looking so far in the future with everything but also this is very conservative country with their rules and habits. If Microsoft would like to look at Japan as on another column in Excel, as they looked in the past for sure, that's really wrong. SEGA is a great example. Ex-American company which is doing well in Japan now. Microsoft shouldn't think only about money and income if they really want to hit Country of the Rising Sun and reach some success. I know, this is important. But there are some more important things in their culture. If they would hire good managers who understand that marketplace and Microsoft will respect that, they could do something good. And at the end… I don't think that Japanese company like that would be sold by American one.