What survivability? Naix is 25+2.4 Strength with something like 1.3 base armor. Leoric is 22+2.9 Strength with 3.5 base armor. Naked level 25, Naix has 2088 HP with 10 armor, while Leoric has 2250 with 12 armor. Leoric is inherently tankier than Naix.
Sure, Leoric will need to pick up a BKB against competent teams, but BKB is 4000 gold. Naix is going to need to buy a Buriza to keep anywhere near Leoric's damage output, and Buriza is a lot more expensive.
LOLLiterally the only advantage is infest ganks? SK doesn't get ganked because he just fights back and kills everyone? I'm officially done talking to you for now because we clearly play on different levels.
Yeah, I'm thinking of a teamfight. Lion's only going to have time to start sucking on Leoric after he's already used his other spells (hopefully on enemy supports?), and there's not much he can do to stop Leoric from just walking up and punching him to death. Best case scenario would be using his spells on Leoric and sucking away while Leoric's stunned/hexed, but then he's not using his stuff on squishy supports and whatnot and Leoric's doing his job of being a mana sponge.
Like I said, it's 200 mana over four seconds of channeling, and without any stats at all, Leoric can still throw a stun, eat a full duration Mana Drain, and still have plenty of mana left over for his ult, assuming he's at level 11 or close to it. It's not unreasonable to think Leoric will be around 9-11 by time teamfights start showing up for real and not just little skirmishes here and there, and pretty much every Leoric build incorporates at least a Bracer and/or Wand.
Necro's been nerfed a lot and will probably continue to be nerfed. It's an item that scales exponentially with how well the player controls the minions. With 0.75 BAT and the feedback on swing, plus essentially an extra Drums aura, it's an extremely potent item that's pretty reasonably affordable. Hell, Drum of Endurance has been getting nerfed almost every damn patch and it's still made constantly. It's just a really, really good item. Necrobook is in the same boat, especially at higher levels of play (where people will do more than summon the creeps, attack-move, and forget about them.)
I personally feel like removing Midas as a means of removing the creeps was a mistake. But, hey, maybe we just need to pick Chen/Bambi if we're worried about Necrobook plays.