Why was Noach "forced" into the teivah?

I have always wondered why Noach being forced in to the teivah by the flood water should be counted a flaw. Why couldn't it be that he waited until the very last moment for the people to do teshuvah? Couldn't it be that he was like Avrohom praying for S'dom up until the final decree? This seems to be the reason why we say "forced by the flood waters". When Hashem told Avrohom "there are not 10 tzadikim", Avrohom stopped. He saw that S'dom had passed the point of no return and the judgement was final. Noach saw that the rain had already come, the "light rain" (from the medrash of hashem giving the last chance to people) had passed, the heavy rain had come, the flood waters had started rising, the last opportunity had passed. Yet he still waited until the flood waters had "forced" him into the taivah. This showed that he was not waiting because he wanted to hope that people would do teshuvah at the last minute.

The medrash states that the word for everything perishing (vayigva) implies being "put to sleep" rather than being left to drown as the flood waters rose. By the time Noach had been forced into the teivah, he would have seen his and known that there was no point in waiting.