Why not use a timer at sender instead of increasing transmision time in CSMA/CD to detect collisions?

In CSMA/CD protocol in order to detect collision Transmission time should be at least equal to two times 2×Tp, that means we have to change the frame size accordingly. Instead of all this why can’t we just place a timer with a time limit of 2×Tp, If there is any collision before the timer expires, then we can come to the conclusion that our data is corrupted, and we need to retransmit. This sounds easier for me as we need not append any additional bits to the frame, Correct me if I am wrong.