Starfleet may have many different classes of ship, but they fulfil different roles and duties. Add to that the fact that with many larger/newer classes there won't be many of those ships commissioned straight away, they would take time to build and deploy (I'm sure that there was backstage information that said there were only around 10 Galaxy-Class ships built over the run of TNG; whilst there are at least only two Sovereign-Class ships, the Sovereign
and the Enterprise
As older ships were retired from service newer ones would take their place (Oberth and Nova). We also have no evidence that shows the Prometheus-Class was a success since it was the only named ship of its class (the one seen in "Endgame" could have been the Prometheus
, pulled from mothballs to meet the threat entering the Sol System).