Well, if the G36 is so great, why design the 433 as basically a metal receiver version of it? I personally think that reports of the G36's overheating issues are probably exaggerated in an attempt to get 416s or something else into the hands of the German Army. We have to remember that the Beretta ARX-160 has about as much metal to polymer in it's receiver as the G36 and the Italian Army and others have had no complaints about it's performance, and it's more accurate than a lot of off the shelf M4s.
Both Ian McCollum and Larry Vickers have made references to these issues in regards to the G36 and the XM8, and Ian especially has said that it does go both ways in that debate and he considers the G36 and XM8 to be good rifles.
Fact is that HK do seem to be wanting to favor stuff like the 433 and 416 as they're seemingly a bit more modern than the G36, though they still produce and market the G36. So I'm guessing that reports about the G36's issues are probably exaggerated. Also, when used as doctrine until recent years recommends (mostly in semi auto), the G36 doesn't seem to have problems. That being said, the G36, at least in my opinion on the outside looking in, has some redundant features and some things that can be better integrated into a fresher design. Also, not every wants the AR-15 on steroids that the 416 is.
I do wonder, though, how much of the issue surrounding the G36 is fact, and how much of it is conspiracy.