I don't know why anyone's assuming we can swap out a regular for a rapid in the future when we previously saw that medium shields can't be upgraded to large shields during refits. We'll only be able to upgrade single shot launchers into single shots that can fire newer and improved torpedoes, assuming new torpedoes even require new launcher setups. They might not!
Partly it's wishful thinking, yeah, but there's good reason to believe that- even if not
directly equivalent- the RFL's volume budget is much closer to a single standard launcher than it is to multiple. Remember, it was developed
for the sole reason that enough standard launchers took up entirely too much space for their salvo size. Given how cheap hull size is, the Federation would absolutely not be paying ten times as much for an RFL that's "merely" half the volume of three standard launchers- the juice just wouldn't be worth the squeeze. They'd have either kept working on the RFL till it was compact enough and/or cheap enough to be worth using, or scrapped the idea and gone back to batteries of standard tubes. Since they
didn't, we can assume that the RFL is of
broadly similar size to the standard launcher. It's certainly not a simple plug-and-play upgrade, but I believe it's a very, very plausible one for a major yard refit.
Another point I just thought of, a reason we might want to go for quad nacelles: I think our warp drives tend to be a smidge faster than the latest Klingon drives, but only because they penny-pinch a bit. Sayle said their warp coils are straight up better
I believe our core is slightly better, but yeah, their nacelles are way up on ours at present, and while we might have a brief moment of superiority immediately after our new nacelles come out, I think their next-gen warp drive tech will be coming out very shortly after that indeed. The tech gap is shrinking, and it's shrinking faster in warp tech than weapon or shield
(actually I'm not sure we're closing in on them in weapon tech at all, but that's a worry for another day), but it's got quite a ways to go.
Edit: To clarify, in real-world performance I think our warptech is already broadly on par with theirs- remember, the Callies were
dramatically, decisively faster than anything in their inventory. Just remember, they're getting the-same-or-better-but-mostly-better cruise and still-pretty-respectable sprint off of a worse warp core, which says
really good things about how power-efficient their warp coils are. If they decide to just throw antimatter-efficiency to the winds and build as oversized a warp core as they need to power those coils as hard as they can take, I expect we're in for an unpleasant surprise- even if they'd be driving antimatter consumption through the roof for the privilege.