I feel there should be a distinction between bosses and trash here, trash imo can be nerfed back to blizzlike values tomorrow. For bosses though seems too early to tell still, most lack tier3 (our paladins for example have 0 or 1 piece still) and like Sandown said earlier, 'nerfs' naxx weekly in itself. But I don't see a reason of prolonging raids by having an increase in trash HP and having all guilds struggle to fit it all in their shedule, Patchwerk takes ages to reach at this point. Other than Sapphirons auto attack damage (the aura _is_ the mechanic, and that buff alone is enough) I wouldn't decrease any damage/HP of a boss at this moment in time, and see where we're at in a month or two, when people actually gear up.
Guilds will die and merge after Naxx anyway, it's the inevitability of endgame content being finished. I don't feel like you can use the fact of people quitting as an argument Naxx is buffed too much, those people would quit in any case. But that isn't to say you should ignore all guilds being in trouble completely, with this population and lack of influx it's not really easy to say just 'deal with it', nerfing trash is surely good direction to take at this point, much more so than re-implementing worldbuffs, I think there's not many people that really liked that playstyle or being 'reliant' on them, and all the toxicity that they brought during raids of not wanting to lose them.
Nerfing trash should be the immediate step, and respond to what happens then later, with either bringing bosses HP back to normal or not doing anything if it turns out t3 makes a difference.