Part of the reason Garen isn't good right now is because many of the champions he's supposed to be targeting, i.e. marksmen and mages, have too much access to persistent CC, namely Frozen Mallet and Rylai's, and therefore get to kite him far too well. The larger issue, though, is that Garen is a juggernaut that really gets countered hard by kiting: while all juggernauts are meant to be kiteable, every other member of the class has a tool that lets them still do stuff from a distance, usually via crowd control with their W or E (or, in Udyr's case, Chilling Smite, and Rylai's + E/R for Mordekaiser). By contrast, Garen only starts to be a threat when he's right on top of you, and despite the mobility and slow cleanse from his Q, it's pretty difficult for him to get into combat easily, let alone reach the back line and ult the Villain.
Personally, I think Garen needed a slightly different direction for his Juggernaut Update changes: bar his upgraded E, which is a pretty awesome ability, all of his other mechanics still have some issues. His W bonus resistance gain is kinda nice as a teaching tool, but ultimately gives power that learners will find hard to appreciate (30 free armor/MR only sounds good once you're experienced enough to know the value and impact of those stats), and that would be better spent on more interesting effects (the W active is also pretty boring, and lacks immediate impact). His Q is simple and satisfying to use, but the mini-windup still doesn't feel great, the silence tends to make him more binary (he either reaches you and kills you without you being able to do too much to escape, or he doesn't and can't do anything), and prevents him from being as good at killing as he deserves to be, and the movespeed bonus scaling in duration, rather than strength, means this ability becomes stronger without feeling as powerful as it should (as with his W, it also limits his ability to feel really good when using Q at the right moment). The new Villain mechanic was intended to give him a niche, but didn't really change any aspect of his gameplay: sure, he gets to be strong against a particular high-value target, but his inability to reliably make use of that means he might as well ignore it most of the time. Even just making the Villain mechanic extend the range of Garen's ult against the Villain, instead of providing bonus true damage, would've helped him out more (not that that should be the answer, though).
Tl;DR I think Garen's unsuccessful right now, especially in pro play, and will remain the same in the future, because his kit rework didn't really address his core flaws, and instead gave him some new mechanics that, for the most part, didn't really end up working out. He still doesn't really have a niche (or, at least, he can't really put his niche into effect reliably, which is what's locking him out of pro play), and if he gets kited (which is especially bad now, what with the popularity of permaslow items), there is literally nothing he can do against his opponent (and, on the flipside, if his opponent can't kite him, there's nothing much they can do to stop him). He needs a way of affecting fights even from a distance, and he needs tweaks to his combat style to make fighting as him and against him a little more interesting.
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