Let's discuss Combined Arms as a concept.

Infantry is incredibly strong, but vehicles still have a solid role. For just a few examples, I run a radar/cas Valk and keep flanks clear, provide radar, and drop squad-mates anywhere they need to go. Kobalt/Radar Ants are OP in their current state. Hesh tanks not only clear points, they suppress spawn rooms and control lanes. Air is in a pretty bad spot due to flak being so OP, but those quick hits are often just enough to let friendly infantry advance to a better position. Tanks are often the only thing that can take out enemy spawns. Vehicles absolutely have a solid place in the game.

Rejecting the reality of weapon hierarchy is not a good way to understand combined arms.

Pretty much that sounds as if vehicles should have the following roles:
1) Transport infantry
2) Provide radar coverage for infantry
3) Suppress (not necessarily kill) infantry
4) Allow for friendly infantry to "advance to a better position"
5) Take out enemy infantry spawns

Which boils down to:
Vehicles are there to support infantry

And that's not combined arms, because really vehicles are not required at all. Each of those 5 things can be done by infantry or their basic support vehicles:
1) Redeploy provides free, instant, and uncounterable movement of infantry
2) Infiltrators can provide radar coverage with no cost
3) Infantry have a variety of small arms by which to suppress, and most spawn suppression is caused by infantry
4) See above
5) Infantry are well-equipped to take out enemy spawns as well, particularly if those spawns are in an urban area (as they usually are)

Just think about two platoons with two squads each (24 v 24). One platoon has 3 MBTs, 3 Liberators, and 12 infantry. The other platoon has 24 infantry. We both know which platoon is going to come out on top (or more realistically, the combined arms platoon is just going to ditch their useless vehicles and things will devolve into pure infantry combat).

I'm just going to quote myself here:

Infantry is a little OP at the moment, but a few vehicles in a few situations are worth their weight in gold.

• Cas Valks
• Radar/Kobalt Ants
• Hesh Tanks
• Anti-Infantry Magriders

Spawnroom bursters and Strikers keep aircraft from playing a meaningful role in many fights these days, so they can only create windows for infantry.
They do support infantry, and can't get much done on their own, but that's fine. Vehicles are still valuable assets, proven by how often they'e pulled in clutch fights.

One thing I do feel is worth mentioning is that, thanks to the largely fixed battle lines and continual supply lines of planetside 2, if your going to compare to real world military architecture, the best overall comparison is ww1 or to some extent ww2, not the modern day. While the equipment may seem reminiscent of the modern day, the ways in which fights manifest are not equivalent to modern military combat, and they instead bare far more in common with the trench warfare of the first world war.