face roll mp 10? well barbs are the only true face rollers. as @oscar, (i think?) is the one who made the video proving this. this will change.
can you survive mp10 with legacy nats? yes. a good player can survive with any gear. viable does not mean godly or fastest clear times. it means passable, doable, able to achieve with. anything is viable. if someone wants to crawl thru mp10 thats viable with the weaker gear they want to use making it viable lol. softcore shouldn't even think about viable. our characters respawn! is l4n endgame? depends on the user and his/her goals.
thats where this whole conversation is kind of broken. change topic to - will nats legacy still be fun and provide speed farming? or will I still have insane disc regen from the set? well yes to the regen unless blizz jus outright changes the set.. which i don't think they're doing. will l4n still have a niche market? yes.
hopefully those people in their "junk" sets get to reroll a stat and create some interesting personal gear. I'm sure they're having great fun running in the junk gear using plenty of disc level grinding to their hearts content! good for them!