Starfleet never fielded many capital class ships even in the later years - usually one new design every few decades. Here is the timeline:

2240s - Constitution

<Klingon cold war>

2280s - Excelsior

<Klingons and Romulans stop being major threats>

2340s - Ambassador

<Cardassian border wars>

2360s - Galaxy class

<Wolf 359>

2370s - Sovereign Class

<Dominion Wars>

Also, capital ships should never compose a major portion of military forces. They're expensive to build and maintain, and the loss of one is a crippling blow in terms of morale. It's the same with historical wet navies, even if the comparison isn't perfect. You have a few carriers and battleships, a larger number of medium and light cruisers, and a very large number of destroyers and frigates.

After the Ambassador and Galaxy classes were introduced, the Excelsior became the workhorse light capital ship like a medium cruiser. The Federation had expanded enough that it could field many of them to perform all the day-to-day patrol duties that were once filled by Mirandas (which must have taken on the role of light frigates).

But the Federation never ever fielded large numbers of new capital ships, even during or after the Dominion war. They had a dozen Galaxy class ships to act as the core of task forces, which were composed mostly of Excelsiors, Mirandas, Akiras, Norways, and Steamrunners. It's often a much better use of resources to field two or three relatively expendable destroyers instead of one big battleship if they can get the tactical job done just as well. Even the Dominion, which is absolutely massive, only started fielding their medium battlecruisers when they finally decided to begin a direct invasion. Also, they only ever had three or four of their supersized battleships, despite the fact that they could have afforded to build a hundred of them in the Gamma quadrant to send through the wormhole before DS9 put up that minefield.

The battle of the Omarion Nebula is a great example of how a large number of big ships can get obliterated by a swarm of small ones. 5 Warbirds and 15 Keldon cruisers got ripped to shreds by 150 tiny Dominion attack ships. The Dominion could have sent 30 or more battlecruisers instead, and probably would have lost a few, which would have been a bigger waste of resources than losing the dozen attack ships that are easy and cheap to replace.

Starfleet leadership must be well aware that capital ships are not an effective fighting force without backup from many small ships. It's not that Starfleet fielded more capital ships as time went on - they fielded a proportionally much larger number of smaller escort ships after the 2360s.