If you get to Admiral and you find that you preferred a lower tier ship i.e. Defiant you should through either the Cryptic Store or some other in game means, be able to upgrade it to a T5 ship. So you'd gain another BO and a bit more hull or an extra console but still play to the core strengths of the ship. So an upgraded Defiant shouldn't be more maneuverable and have the same amount of hull as the T5 escorts. It should still be a choice of say maneuverability over say a bit more hull or more crew. Hope you all get the idea.
This way, we won't all be in the same type of ships and the lower tier ships would get some use beyond the initial grind.