There are very few airlines that adopt this approach, precisely because most airlines fly routes that vary significantly in the frequency required and number of pax that need to be carried. For example, BA
operate a single daily flight MAN
using just a 767 and 7-8 daily flights LHR
using mainly 747s with occasional 777s. They have these multiple a/c types to fit this mixture of capacity and frequency.
Of course there would be benefits to only having a single type of aircraft, but the disadvantages would far outweigh them.