Actually, yes, frontlines aren't very great right now, atleast in high elos.
It's all about getting early map-controll, and for that you need a strong early jungler and dominant lanes. Frontline champs usually have a not that great early jungle(the best you get is reksai, and she's not that great as frontline) and generally have issues in lane vs ranged champs and rumble, which are very popular.
Same bot, if played well, 2ranged>ranged+melee at bot.
And for teamfights, if your damage comes from far enough and you still have cc in the setup, playing without frontline is quite possible, though harder to execute. But if executed well, it's just stronger to have a mostly damagefocused comp, cause frontlines die way to fast later on.
If you whatch the SKT vs IM game(http://www.lolesports.com/en_US/vods/worlds/world_championship_2016), you can see how a well played squishy comp shits on frontlines, at some point a** full armor** 3.5 items nautilus gets vaporised within the 2sec of an varus ult vs a full AD comp...
However, since full squishy teamcomps are harder to execute and get punished harder for mistakes, going for squishy comps in soloQ isn't smart, especially in low elos. Which is why tanks usually have a good winrate, cause for soloQ haveing a frontline usually is a big advantage.