He's certainly not useless, and his bullying as well as early and mid game presence is terrifying when played well. His bad matchups won't dominate him, and Pantheon still retains a presence on much of the map. Micromanagement of matchups isn't accurate as it does depend on skill levels of players and coordination between them. If you mean to say abilities that hard counter another champion, then an advantage is acceptable to ascertain, but certainly not a definitive decision.
At late game, he isn't the best of course and falls down quite a ways -- however one must note that almost nothing falls off to an ADC or squishy AP. Late Pantheon will still be capable of 100-0ing one of them considering how fast his combo can be done. His walling by going in and posing a threat while simultaneously incapacitating a carry for a second is game changing, even more so if he lands that kill and expends some abilities of their team to peel him off as your team coordinates with him. Not to mention his pincering effectiveness which is a very strategic tool for teamfights.
Pantheon has a very unintuitive kit and extremely straightforward // aka boring. Point and click damage, point and click stun, AoE channel. Ult is the largest portion of his arsenal to impact a snowball, but it's a bit difficult to land or utilize to the full extent without some coordination, or luck. Managing his passive is important too of course as it can make an otherwise impossible dive to get out of alive, very possible.
He won't be in pro play too much (only was before this season due to his E being a little over the top in terms of just insta jump killing a squishy while still being relatively tanky -- problem like we had with tank assassins), but toning that down -- he still has the attributes that work very well in an organized team, or even mildly cooperating for his strategic cut and/or engage.
So the point that's here is more: Do we rework a balanced champion just to make him more satisfying to play?, because his playstyle is linear in most parts and with little counterplay if he catches you out of position? or do we rework him because he is outdated in terms of skill? Or do we just leave him as is?