Neither series failed. They both had more viewers than TOS. There were complaints from hard core fans but the shows drew in new viewers that they wouldn't have if they had done as you suggest.
Secondly Enterprise was made stylistically to be older and lower tech than TOS. it had the same design traits but was "chunkier" and less smoothe in design. Meanwhile Voyager was a top secret experimental ship with new experimental tech that got lost. All the other ships of the era remained consistent