The main problem is having a LAC that survives to get that close to a live capital ship that needs that kind of damage in the first place. And, again, remember that with a standard nuke you need, in effect, a contact explosion. These can be obtained, especially if the launch range is short enough and/or the target's movement is sufficiently predictable and you can get through the active and passive defenses, but the odds aren't going to be good however you package it. A LAC is a very vulnerable carrier to get to the sort of range where a contact hit would be probable, even in "squadron" level launches. A single hit from a laser head (whose chance of hitting the LAC will be orders of probability greater than the chance of the LAC hitting an all-up starship with a contact nuke) will kill the LAC, so you find yourselves back in the days of the "Sonja Swarm," trading dozens of LACs for the off chance of getting one of them into range to inflict damage on the target.
Further, you should recall that bow walls and stern walls are becoming standard features of warships on both sides and that Rafe Cardones couldn't stop a battlecruiser with two nukes that got through the ship's sidewall. He hurt it a bunch, but he didn't kill it. So what's going to happen when you get your single warhead through the sidewall (or bow wall) of an SD, with its vastly thicker armor and better rad fields? Laserheads are used not simply because of their stand off range, but also for their focused ability to burn through those passive defenses.
And, while I'm on the subject, there's another thread wandering around here about refitting capital ships. Someone (I don't recall who) commented that there doesn't seem to be a lot of that going on in the Honorverse. I beg to differ. Where do you think all those new style compensators went/came from? Or the bow wall/stern wall add-ons, or improved EW capability, or the improved counter missiles and decoys, or the FTL coms? There is a constant, ongoing upgrading/refitting process in all of these navies. What there is not is what amounts to rebuilding of warships.
Let's not get to caught up in comparisons between the StarFire universe and Honorverse. There are entirely different constraints involved, not least because of the fact that the StarFire novels are so intimately tied to a specific set of gaming rules which, in their original form, made refits that were actually rebuildings entirely too easy, IMHO. When I got involved in the reediting of the rules, I made things a bit tougher, but not much, because the refit/rebuild process had become so much a part of the "flavor" of the game.
Nonetheless, the fact is that throughout history the sort of "refits" people are talking about when they talk about "upgunning" the older SDs amount to rebuilding. And, historically, people, it's been more efficient to build new than it has been to substantially "up gun" old ships. The fact that most major navies spent a lot of time "refitting" BBs during the period between the Washington Treaty and WW II sometimes inclines people towards assuming that this was the standard practice, but it really wasn't. It was an aberration forced upon the navies in question by the prohibition against new construction embodied in the Washington and London naval treaties. IIRC, the only navy which actually upgunned in the process was Italy, which completely altered the arrangement of some of its ships' main armament and replaced the old guns with new weapons, but most of the upgrades were confined to better (or, at least, thicker) armor, new propulsive machinery, improved secondary armament, and the transition to oil from coal fuel in many cases.
Refits of this nature have, in effect, already been carried out by the RMN with the shift in compensators and the improvements in EW and communications equipment. It is not physically possible to "refit" existing SDs as pod designs. It is possible (although extremely expensive and time consuming) to refit them by removing their old broadside launchers and refitting a smaller number of MDM missile tubes. In the process, however, their magazine space will also have to be ripped out and completely rearranged, including new high-speed ammo handling equipment to move the (much) larger missiles to the launchers. Basically, you'll be removing a triple-14" turret and replacing it with a double-18" turret, with an accompanying 1/3 reduction in ammunition stowage. The time it will take to rip that much of a ship apart and rebuild it would be far better spent in simply building a new one from the keel out. You could probably build half a new ship for the cost of the "refit," and the yard time involved would probably be no more than 25%-30% greater than that which would be required for the "refit." Why tie up yard space producing a stack of SDs which will not be able to remotely match the rate of fire a proper pod design could when doing so will set back your new construction projects by, literally, years? Far better to think in terms of some sort of "strap on" pod launcher system such as the Andies came up with on ships which are otherwise pretty much unchanged.
Note that some of the late model pre-pod ships in the RMN and the GSN are in a somewhat different category from their older consorts. Some of them were built after the probable dimensions for the MDM were already known, and they were given larger launchers, bigger magazines, and outside ammo handling equipment while still on the building ways. Those ships need no refit to handle the MDM through their broadside launchers. (Think of this as the USN's decision to design "outsized" torpedo tubes into its latest generation of attack subs so that if bigger weapons come along, they'll be able to fire them from existing platforms.) In addition, some of the Ghost Rider tech was engineered backward into smaller missiles which are, in effect, only 2-stage weapons. These are carried by ships like the Edward Saganami-class CAs and their contemporary BCs, which didn't have the hull capacity for all-up, full-scale MDMs. And all current-build RMN and GSN missiles have improved drives, even if they are only single-stage weapons, courtesy of the research which went into Ghost Rider. Which means that they have superior acceleration and better powered-attack range than their opponents.