I've always been a bit funny about the seemingly flimsy warp nacelle pylons we have seen on the Enterprise-J when it was first shown in ENT:Azati Prime. Structurally it didn't appear particularly strong though it clearly did the job as showcased during it's battle against the Sphere Builders. In Discovery, he have seen how in the 31st century they make wide use of the "detached nacelle" technology to have them physically detached from the ship. While power transfer from the warp reactor hasn't been fully clarified how this works to my knowledge, it stands to reason there is some kind of mechanism that allows for this while having them fully independent of the main ship.

I'm proposing that the slimline struts of the Enterprise-J are an early evolution of this technology. It would explain the lightweight pylons we have seen connecting them to the main ship body so perhaps this was an earlier development stage of the technology where the nacelles were already functionally independent of the main body of the ship and not dependent on it for structural stability etc. But for all intents and purposes they still hadn't quite cracked the "wireless power delivery" part of the design yet hence the nacelles still requiring a small physical connection to the main ship for power conduit purposes but not as a load bearing structure. This would go a long way to explaining the unconventional design of the Enterprise-J in that the nacelles were more or less independent of the main ship which would be a boon for safety, operational and replacement purposes but until the development of true connection-less power delivery technology like we have observed with the 31st century design there was no way to "fully" disconnect the nacelles from the ship itself at the time.

tl;dr: The Enterprise-J and it's unusual design was an intermediate step that ultimately lead to the development of the true detachable nacelles seen by the 31st century.