An actual technology demonstrator is usually pretty useless for anything else. A Gloster E/28/39, the British tech demonstrator for jet aircraft, could never be turned into a fighter. There was just no provision for many essential pieces of equipment, like a radio, guns, or cockpit heaters. It was built to do one thing and one thing only: figure out how to make a jet engine fly.
As for a prototype: well, the lead ship of every new class is always a prototype. It's a new way of assembling things; there will always be lesson to learn and places to improve. If a ship class gets abandoned after one or two ships, sure, the entire class could be considered prototypes. But a non Galaxy-class ship isn't going to be a prototype for the Galaxies except in the most general way.
I broadly agree with what you're saying, of course, since it's in general terms the same as what I was saying. But I think it makes more sense to conceptualise each ship as something that was intended to be fully capable and functional - which tech demonstrators and even prototypes often are not. Perhaps a better metaphore is the dreadnought-type battleship, as seen in the Royal Navy:
HMS Dreadnought is a class of one, built to prove the concept worked (so fits with your idea, Tim,) but also built to be an actual functional warship, and she did fight in WW1.
She was followed by the Bellerophons, a class of three. These are broadly similar to Dreadnought, and in Trek model terms would be like the TSFS Excelsior vs the TUC Excelsior, so we here would probably lump these together into a group of four as a single class.
Then there are three St. Vincents, one Neptune, and two Collossi, before things settle into the second generation Orions and King George Vs. In each case, the ships were intended to be actual ships of the line, serving as any battleship did - but each is also a slight change in design, a tweak of ideas, an experiment to see "will this work better." Thus the RN entered WW1 with something like 7 different classes of battleships, which in Trek terms would have 4 or 5 different physical models (the other classes being minor variations on a single filming model).
This is, of course, quibbling about semantics. But isn't that what we do here? Anyway, this has definitely entertained me while I wait for food to cook, and I hope it's entertained you too!
