I see and understand that some divers wear their cylinders very high on their backs on sub-optimal doubles plates. And I understand also they do this, intentionally, for their trim needs and also because they might have better access to the valve for whatever reason. In such cases a center outlet wing would likely not interfere with the first stage because it and the center outlet would be positioned well below the stage.
However, not everyone wears their tanks hiked up behind their heads, I do not, my wife does not, most of my usual dive buddies (with exceptions) do not. And again, we do so for trim, for comfort as the stage does not hit my head and if need be I can still get to the valve though admittedly it is slightly more difficult if that is a concern.
This is my Oxy 18 on an Oxy Ultralite with a Mark 17 Evo. The Oxy is a side, not center outlet. If it were you can see that both the stage and the outlet would indeed interfere. And this is how I wear my tanks (for both trim and comfort and access):
And with a Mark 2 Evo:
Now, true, I could lower the wing and the results would be the cylinder very high on my back and the first stage bumping my head and destroying my trim. Because we are all different and what works for some may not for others.
(I have all the equipment scattered all over the living area because I have just completed three consecutive dive trips in a row, two of them lengthy, and it is time for maintenance of everything. My wife will be glad to have her space back
.)
And I want to point another important thing. Most wing and plate users are using doubles plates. Doubles plates require an STA for stability with singles and even if you get away without a STA the cylinder is still positioned very far away from your body and alters the CG and CB (center of buoyancy). Doubles plates, since they position the tank sub-optimally so far from the diver, it does have a side benefit, albeit, of allowing more space between the wing outlet and stage.
I do not use doubles plates for anything but doubles. I use single tank plates, the Freedom Plate, the Oxy UltraLite plate and the VDH plate and similar. The advantage of a singles plate is the cylinder is very stable and does not try to roll side to side, the center of gravity and buoyancy are closer together, the divers trim is better and it is more comfortable and compact and thus more optimal to use a single cylinder with a plate intended actually for a single cylinder rather than a doubles plate adapted to a single cylinder. But it does crowd a wing with a center outlet right into the first stage, thus my preference for a side outlet wing like the Oxy and VDH wings.
Using a doubles plate for a single cylinder reminds me of the Snickers Bar episode in Seinfeld. One person saw another doing something wrong and thinking it it right did so too. Then another copied him and his buddy copied him and then his wife copied him and then another bunch copied all of them and each other until wrong became right. Or maybe just being tight with the cash and making do with what they have. Just saying.