You can do that.
People generally didn't do split runs in Vanilla. It would stand to reason that if you took current days model of raiding and applied it to the whole of Vanilla, these guilds would have legions of geared characters through the entire development cycle and eventual launch of Naxxaramas. You know what one of the underlying criteria was for defeating four horsemen? Generally you wanted a lot of tanks with T2, and in some instances you would want to farm the stupid Hydra boss in Zul'Gurub for the trinket that essentially meant that your taunt couldn't miss. Four horsemen was difficult, but a lot of the underlying difficulty was because a missed taunt (miss chance being 8.6%, without any % hit IIRC) would basically cause a wipe.
People didn't necessarily have to grind Naxx exclusively in order to progress through Naxx. Most of the progression and prep would take place before Naxx even came out. In this make believe hypothetical world that this question is being asked, it would be assumed that if a Naxx would be dropped today, raiders would be far more prepared for it than they would have been in the past. How many guilds did several split runs of AQ40 or BWL prior to Naxx coming out? Probably none. You're not taking into account how prepared people would be by today's standards.
It also wasn't common for people to use world buffs (Onyxia, BRS, ZG or form Felwood flower groups) until late into Naxx. Could you do it without? Yeah, but most guilds would save as many of these as they possibly could until the end of the week for Loatheb, Sapphiron and Kel'Thuzad. If guilds ran into problems early, you can be rest assured methods like this would be abused far earlier in the instance.
Game is also much more open now, there really aren't as many true contenders for competition at the very top like there once was. Poaching and leaving guilds happened back then, and in the event there was gear checks that dozens of split runs couldn't overtake, you can be rest assured that people would hop around. I mean look at guilds taking mages who had the legendary staff from Firelands for spine in Cata?
I think some of you are vastly underestimating how much people raid now compared to then. As painful as it was with 40 people, you could be rest assured that in this scenario of a Naxx dropping that guilds using today's mindset would have several or more alternate characters running through MC/BWL/AQ40 in preparation. It really doesn't matter if there is one difficulty to each instance, you could still do it.
I hope some of you realize that the only DPS worth a damn in vanilla was rogues, warriors and mages. Rogues and warriors being bargain basement as far as getting raid worthy gear was concerned. Aside from weapons a lot of the best trinkets were from 5 man dungeons (hand of justice was essentially top tier the entire time, and the 2% crit trinket from a quest could hold up the entire time too) and in the case of warriors just random shit strung throughout every raid, some of the shit they would use was crafted. Basically gearing the only DPS that matters wasn't particularly difficult. Even if they were shitty pieces of armor you would still want to run with +5-10 weapon skill because of how glancing blows worked, with rogues being upwards of 70% of their damage coming from auto attacks. Didn't matter if another rogue in your group was decked out in AQ40 gear, if you were a rogue and you had +10 weapon skill you would dump truck their damage because they didn't know how to properly gear. Edgemaster handguards ring a bell to any warriors back in Vanilla?