We already have a hammer champion.
But that's just 1 other hammer champion.
We have over 20 sword champions and 30+ 'monster hands' champions.
There's certainly room for 2 hammer champions.
Also I wouldn't call that thing J4 carries around a lance. That being said, we've yet to attain a lance champion and out of all the champions you could give a lance too I find a Knight to be the most fitting.

All these guys have a weapon that would be classified as a "Lance" if the lance classification existed in a typical RPG scinario (like Fire Emblem's Lance/Sword/Axe dynamic).
There's also the javelin and trident wielding champions in:

While there admittedly aren't any historically accurate medieval lance wielding champions, there aren't many historically accurate weapons in LoL in to begin with.
Though some would argue it wouldn't really feel like taric and that the hammer is iconic to his theme. Eager to see the communities take on this...
I really feel a hammer would fit Taric better.
Very similar to Nautilus: Taric's aesthetic is all about being the slow yet sturdy character, communicated through his character design, animations, casting times, and abilities themselves.
Having a mighty hammer really fits well with this aesthetic.
A lance does not.
Spears (and related weaponry) generally communicate a sense of speed and weight that's somewhere in between a mighty hammer and a swift set of daggers, being equal parts about strength and speed rather than being married to one specific direction.
This fits Xin'Zhao, Pantheon, and Hecarim pretty well as they're all fighters that are equally reliant on durability and mobility with play-styles incorporating a mix of both. Mostly charging straight at their foes and having the durability for that to work, without being an immovable tank in terms of durability or a slippery assassin in terms of mobility.
I just don't see a spear really fitting with Taric's aesthetic.
As what's essentially LoL's version of a Paladin, a Hammer just fits him much better.