^ Using the US Navy as an example, ship's roles change over the course of time.
In WWII we still had a large number of batlecruisers & battleships and though they tried updating the Iowa class for modern use, more out of nostalgia I think, they really didn't fit what they needed a ship to do in the 21st century.
Plus they were falling apart, structurally speaking and risked major damage every time they used the 16" guns (USS Defiant?).
Same as what you're saying Matt4511
, lighter, quicker ships are the way they're going for the bulk of the fleet.