Why replace Ostuka?
He's just dubbing over Sutherland's performance, as well as some others.
Hayter got replaced because Kojima wanted to go with an actor to utilize facial acting. Whether it was a smart move to replace Hayter or not we'll have to wait and see once TPP comes out, but replacing Ostuka probably wouldn't have changed anything other than angering the Japanese fans. If anything, I'd be questioning why some of the older American cast was brought back instead of getting "better actors" to replace them.
I know on this board we're pretty much split on how we feel about the series, but the Metal Gear brand is still looked on very positively by the gaming community. The last major console release was 7 years ago, supposedly Kojima has been working on TPP for 5 years now, it'll be running on an entirely new engine, the gameplay has been revamped from the ground up, Kojima's reputation within the gaming community is probably at an all time high. I just can't see any parallels between the MGS franchise and the RE franchise. There's a lot of people out there that actually expect MGSV to be the best Metal Gear yet.
I'm not saying you're wrong about how you feel about the franchise and Kojima, you're definitely entitled to your opinion, you're probably a bigger fan of the franchise than me, but I disagree that the majority of the gaming world feel that the series has been ruined beyond repair and Kojima is washed up. I could be wrong though, I'm just basing that on the responses I see from major gaming sites and such.