It would only need such a nerf if it were to be squeezed onto a medium pad.
As a large ship, it's in a pretty solid place; sure, it's nowhere near as good as the likes of the T-9 or most of the other large ships, but it's also 1/10th of the cost of most of them. There's nothing that lets you move cargo as efficiently anywhere near the T-7's price range, the real price you have to pay is being excluded from landing at outposts.
As a medium ship, it should instead be compared to the other high-end medium ships, specifically the Python as it is the only multirole that really pushes the limits of the medium pad (the Krait MkII is arguably more combat-focused and is also cheaper than the Python, so not a good point of reference for trading).
Compared to the Python, the T-7 currently has 16 tonnes more cargo capacity and better jump range because it saves weight on sensors which together make it pretty much objectively superior for almost any kind of trading. Sure, the Python has more weapons, stronger shields and a more powerful distributor, among other things, but none of those extra capabilities make any difference to a trader. Traders don't care about their weapons, they don't really care about their shield strength, they don't care about their distributor, it's all about the cargo capacity and jump range as they are what translate to profits. If the T-7 were to be made into a medium ship with its current statline, it would render the Python obsolete for trading as the T-7 would be overall better at trading and a much lower price point.
Sure, the Python is much stronger at combat, but it's also costs several times the T-7 once outfitting is accounted for - that's the multirole tax there and then. More importantly, having all the guns in the world doesn't matter when you are delivering goods. What's the point in having 3 large mounts if you never deploy weapons?
If an unnerfed T-7 were to be made into a medium, then any serious trader looking to maximise their credits/hour would use the T7 and the Python would be relegated to the quirky second-rate ship that people take out for a spin when they get bored of the T-7. Sure, some people might enjoy the Python and accept flying an objectively worse ship, but that's a matter of preference rather than balance.
However, with just a 16 tonne capacity nerf, the T7 would actually trade blows on equal footing with the Python. The Python could be refitted for combat after you are done trading, but you could afford both an FDL and a T-7 for the price of a Python; the Python has slightly better realspace manoeuvrability, but the T-7 has a slightly longer jump range; the Python is able to shoot down interdictors effectively, but the T-7 has amazing yaw performance (which really helps when avoiding interdictions). With just a 16 tonne capacity nerf, the question of "which is the better freighter?" would remain a point of debate, rather than unanimously being answered by the T-7.
The point of balance isn't to exclude things from top-tier viability, but to make sure that many things are operating at top-tier viability.
