It's not just planets that don't have large landing pads, there are the orbital outposts too. Many systems only have the little outposts, while quite a few missions go to outposts, so they are an important part of the game.
a) They would be out of order for someone flying an Anaconda too.
b) Does it need a trader?
c) You don't need a T7/9 for any of them
So this problem is extremely moot.
The problem is your claim " As you pointed out, nothing really hauls as much cargo as cheaply as they do" is still leaving that claim I quoted as a "problem". Either it was a problem,or you are avoiding it just as much, so it's yours as much as mine.
And I really do not believe this to be true, even with the system as-is. The Anaconda moves more faster than the T9. And the difference between the prices at that point is one or two runs tops. The money difference is irrelevant.
So with the traders you have no point to keep with them except at the very low end where credits count.
This would be fine if they could do another task really well, but they can't. That may be "fixed" when you buddy up with someone in a fighter to help you, and when they change mining a lot. But as it is, they do only one job well, and to be honest, you can't make do without combat, which these don't do at all.
Price isn't a problem, except in so far as they make a choice going up ranks usable. But at the moment, the difference is very small and the disadvantages of the bricks in actual game play as opposed to "RP" without any PvP allowed make it rather dumb to use the tradeship specialists.
And, as said earlier, the T7 has half the volume but much greater efficiency and no need for space set aside for larger hardpoints and energy delivery to same, plus also the same weight as the Anaconda. So why not up it and/or make it lighter? It then covers the Anaconda which is also a Large landing pad, as opposed to the Python which is a medium?
And at the very top end, you'd pick the Imperial ship, not the T9. Which is daft, innit.
But if you just added slots to make the space, you could use them to make the traders multi-role, whereas if you make them fewer but bigger compartments, any attempt to multi-role would nerf the ship, whilst filling it with cargo racks makes the choice "If you're going to do trading, use the T-x ships" a no-brained "Duh!". But using it for anything else? Another no-brainer: nooooooo!
Heck, you could dump the ship launched fighters too. It only helps if you drop them off to distract whilst your ship engines charge, basically no more than the X-series Fighter Drones: drop it and run like heck! And they're a bit expensive and take a long time to reappear to be used just for that when carrying a huge amount of profit cargo is an option.
EDIT: "needs changing" only if my reasoning is warranted. If there's gaps filled by multi-role and that's percieved as worthy and wants keeping, then no changing needs happening. The T7 had some buffs that from what I've heard have been welcome, so it's not quite so obvious a decision any more, it's just not the shoo-in that I think it ought to be for these specialised and otherwise "crippled" machines. In RP the cheapness may well be enough reason to have NPC freighters use them.