I think this is the most likely answer. The Galaxy Class was also probably already on the drawing boards by the time the Enterprise-C was destroyed. Starfleet probably wanted to save the name for when their massive and prestigious new class of ship was ready.
The Galaxy-class was a Starfleet heavy capital ship class first introduced in the mid 2360s. It was one of the largest and most powerful Federation starship classes of its time, with many serving in the Dominion War. Starfleet had previously operated another type of Galaxy-class starship in the...
memory-alpha.fandom.com
Although the
Star Trek: The Next Generation Technical Manual portrays a long, multi-decade design and construction of
Galaxy-class ships, "
Booby Trap" and "
Eye of the Beholder" seem to suggest that this timeline was much faster, as the warp core design was finalized just one year prior to the launch of the
Enterprise-D. The ship itself was still under major construction one year prior to "
Encounter at Farpoint".
The Physical Construction probably didn't span the majority of the 20 years. It was probably the last few years.
It's the:
- R&D
- Virtual Design
- Independent components testing
- Figuring out the Supply Chain
- Figuring out the Construction Assembly Line
- Doing the integration of all components into one co-hesive package.
- Shaked down of the ProtoTypes to see if it meets basic working function.
- Testing of the ProtoTypes to see how far it can go & what are the limits, do they meet virtual predictions, or can it exceed them?
- Refinement of the ProtoTypes
- Working out any bugs in the system
- Numerous Miscellanoues Steps that I'm sure I'm forgetting.
- etc.
There's ALOT to do when making something that is the "Best of the Best" within the UFP / StarFleet at the time.
Especially a "Top of the Line StarShip". Gaining the institutional knowledge to build soimething like this could be a persons major life-time work.
Upon its launch, the Galaxy-class had become the most technologically sophisticated and complicated ship ever built by the Federation.
Look at IRL, the F-35 is the most Cutting-Edge in Stealth Fighter technology, it took a LONG time to get all the R&D, testing, validation, & integration going before mass production can start.
The JSF started in 1995, FRP (Full Rate Production) didn't start until 2021. That's 26 years before Mass Production starts.
So 20 years to make the "Galaxy Class" makes ALOT of sense given what it should be able to do & how it should be the "Best of the Best" at that time.