Attempts were made to 'clean up' the P-40, but because it was niether urgent nor in demand (as in the german case with the Me-109) it was allowed to recede into the history books as a workhorse, not the best, but it did the job that was asked of it. I mean if the japanese had taken the aluetians, and then kept moving on and on towards more american territory very quickly and unstoppably, then maybe we would have pushed an successor through, but it of course would have had some serious shortcomings, like the much later 109s, with the very, very rough high speed handling, short range, and just a slight redesign of progressive airframes with ever-uprated engines. But fact is, new and more advanced fighters were coming out, negating the real NEED and push to upgrade something from the mid 30s.