But SF spent a lot of time figuring out how to retro-fit the old ships which isn't trivial.
It's usually easier to work from a clean slate and have everything made with the latest / greatest.
Don't forget the sheer Man-Power / Volume & Mass of Materials needed to construct each vessel during it's initial manufacturing run.
Negligible, because in the 24th century, automation would likely be applied to R&D for upgrading older designs, not to mention to actual upgrading work (the notion of manual labor in a space faring society such as the Federation is a ridiculous one to be honest - one that would quickly fall apart).
Also, you are assuming that most SF innards cannot be simply swapped out for the latest version.
The innards might require some modification in regard to accommodating new designs, but probably not much... and nothing that can't be done with transporters (just beam that whole section out, convert it into energy, and then simply replicate new one using that existing energy that would accommodate a new shield generator for example).
Materials can (and probably are) harvested from existing ships that will subsequently be upgraded using new hardware that was made from those harvested materials.
According to Memory Alpha:
An Excelsior takes 750 officers and crew to operate and has a mass of 2,350,000 metric tons
The Defiant takes 40 officers and crew to operate and has a mass of 355,000 metric tons
So for one full Excelsior Crew Compliment, you can have 18.75 Defiants Crew Complements filled out
So for the raw material of 1x Excelsior build from the ground up, you can literally make 6.62 Defiant classes.
Given the same Number of Officers & raw metric tonnage of materials processed into a Star Ship.
That's assuming SF is planning on constructing new Excelsior class ships - which is unlikely given the age of the design.
A more likely outcome is that existing Excelsior class ships in service would be upgraded by harvesting the innards of those ships in question and reconstituting them into new material - a process that will likely result in ending up with excess raw material which gives you technically more leftover raw matter to use in construction of newer classes of starships.
Also, automation would probably reduce the need for large number of personnel as opposed from 80 years ago... but given the size of the vessel, with the latest upgrades, you can send the ship on long range exploratory missions (for which the Defiant is not exactly suitable) similar to those that the Galaxy class does.
That's what bigger frames like the Excelsiors would likely be most useful for.
As I said before, mid-sized and large ships serve other functions besides combat (SF's primary purpose is exploration as well as being the defensive arm of the Federation).
I can have far more Defiants out there doing there jobs for the same Crew Compliment.
I've never advocated for SF to discard larger ships, all ship classes have their purpose and place.
But the fact that you're so dismissive of the "Little Ship" shows your lack of understanding it's place in the Fleet Makeup.
The Defiants would have more limited operational range, and wouldn't be suitable for deep space assignments (unlike the upgraded Excelsior for example, or the Akira, or the Intrepid... or the Nebula class, or Galaxy class).
Besides, I wasn't suggesting creating more upgraded Excelsiors... but rather upgrading existing ones.
I am not being 'dismissive' of the Defiant... I'm merely pointing out that it's not as 'perfect' as some people like it to be.
There's a time and place for all ships, but the High Power, little Package of the Defiant definitely has it's place in StarFleets makeup.
Certainly its better when big things come in small packages, but the Defiant has a lot of limitations in comparison to larger ships outside of battle operations - one shouldn't forget about that.
Don't forget that given Starfleet's large territories that it needs to cover, having many Defiant Classes will help it protect all it's member planets in terms of Border Patrols / Internal Border Scientific Missions.
This leaves larger StarShips free to explore the unknown.
Not saying that the Defiant has no purpose... merely that its technologies and underlying premise can be applied to existing larger ships as well with simple upgrades.
Obviously, SF might have smaller ships like the Defiant inside the Federation rather on the outside doing deep space exploration given their Warp speed limitations (until those are improved upon) and overall size of the crew... not to mention crew comforts that can impact long term exploration (and despite the Defiants size, I really doubt there was a need for claustrophobic cabins and bunk beds - SF of the 24th century would have been able to do better - that was just writers pandering to an outdated premise).
My point about upgrading the existing Excelsiors (not constructing new ones) is that SF easily gave them a 'breadth of fresh air' and can send those older ships out to explore given their overall size and improvements in question and keep them in service a while longer.
If it is accurate that a starship hull design is envisioned to be in service for about 100 years, then the Excelsior class ships would have stopped being produced a decade or two after initially being released into active service... and then those ships that have been left in service (once the design production was halted), they would have been upgraded to remain in service until they reach 100 years of age.