Because for some reason you interpreted "coverage" as the range of a single ship, which is hardly relevant for us right now? We ships because we need them to go to places to do things, and having more ships means we can do more things in different places at the same time. Individual larger ships will have more range if we give them bigger antimatter tanks, but they're going to have similar effective response ranges as anything else with the same warp core, so more ships covers more territory.
Shields scale with size, but I haven't seen anything saying that phasers do too. You can fit more torps, but they're the single most expensive component aside from maybe the warp core.
The Excalibur was not on the edge of viability, it's likely capable of consistently winning a 1v2 against the D7. None of the modules have had any real impact on the tactical capability, which is the entire point of the ship. They'd definitely make it less useless during peacetime, though, and maybe we'd have even gotten a few more post-war. We'd probably be able to build fewer 230,000 ton ships though, seeing as how that's closer in mass to a Kea which was specifically called out as hard to build, as opposed to the Saladin which was adopted because the Kea was hard to build. Not so important in peacetime, a huge problem when it's our only ship that can fight D7s.
The Sagarmatha, Thunderchild, Kea and NX are all larger, and the Saladin ties it in mass. 180,000 tons is, as demonstrated by the Saladin, the upper end of what we might call a light cruiser. It's not small, but it's hardly a large ship. The Excalibur has also been called out as being rather lacking during peacetime, being used as a glorified patrol boat until it was finally allowed to serve as a makeshift explorer. The reason Starfleet adopts dedicated explorers to begin with is that too many Excaliburs die during the Pathfinder Missions, which is certainly a legacy.
And it's just not an incredible feat of design to pick the options that make it shoot good and hard to kill while skimping on things that don't affect that as much. We even made a choice (thrusters) that spent a bit extra to make literally no difference to the ship itself, after we had decided on a half-saucer in part to mount an odd number of thrusters.
I don't recall that coming up? There was some discussion about it during the Excalibur design, but it's not been mentioned in anything threadmarked that I can find. The technology post was also last edited a month after all of that and doesn't seem to mention any phaser scaling, we've since had a vote on phasers on the Attenborough, and the shield scaling post exists. I guess it's possible that it was implemented in the background, but I don't think I've seen it anywhere that would make sense so far, and it's been like two months. Maybe we'll see it soon?
