Agreed. I see no value people writing reviews until its completed. Im for people posting comments ect as the game develops, but theres no value posting a review on an unfinished item. I'd rather have accurate reviews with less of them, than thousands of fake reviews all trolling about something not even completed.
I'll say that reading reviews is far more convenient than digging through a thread, especially in case of long ones, where people get much more into the technicalities, updates or asking how to do something. It would also hurt long running games, and I don't mean milking, but games that do genuinely have long/indefinite development with proper content updates.
The argument is definitely there about reviews becoming irrelevant over time, and there perhaps could me some kind of system about that, but either way restricting reviews to completed games wouldn't be in any way a straight good decision.