New Sylas is just an actually viable version of galio mid.

Aoe Q, check, AOE magic damage autos on passive-check(sylas's is better), Dash w/ attached CC check, magic damage shield check, teamfight ult (dependent on enemy team but) check.

So what makes sylas viable midlane? He actually deals damage so thats a big factor, but the real factor is his sustain, it allows him to actually trade with enemy midlane where as galio just gets bullied out of lane is almost every matchup.