I guess the thing is, if you lose lane as malphite and you can't scale like you want, you're relegated to being a weak splitpusher who is at a high risk of dying unless you use the very ability you want to keep up to be used on the enemy as an escape. and once you go in as malphite, you're in. you had better have followup from the team.
if you get engaged on as malphite while grouped, your ult can be used as disengage... for the rest of your team, but now you're in deep.
peel? lol, ok malphite
I can't think of a single scenario where I'd rather be a Malphite over nautilus right now in top lane. And I like Malphite, don't get me wrong.
As Naut, you can lose your lane and still be super useful in multiple scenarios. You can disengage with R, you can engage with R (and thanks to its long range, you can ult their backline from far away and still have it hit their front line too).
there's also his passive, which has won me so many teamfights from just autoing everything running at my back line and keeping them locked down with Q and E and R, all while doing respectable damage in the process thanks to sunfire/W.
plus you can roleplay spiderman as nautilus. it's so fun. once you have a good amount of CDR, your hook is up so often that you're a nightmare to run away from.
malphite? flash R, that's all you really have in a teamfight. and again, not disrespecting it because it's a great ability, but naut can do exactly the same thing in numerous different ways. had a game the other night where I lost lane as naut, but I was always able to find flash-Q hooks onto somebody important to initiate the fight and we just steamrolled.
more tldr; nautilus is so freaking wonderful to play because he does everything you need in a toplaner in the current meta, and he does it better than just about everyone.
i hope this gushing rant about naut-chan expressed my feelings adequately. zky cute naut pic pls
