Right Not cubes of Partial dTons.
The cargo rules say that the final volume is "X". They say nothing about what form that "X" comes in. It could well be crates or barrels or bales or whatever. That's why that stuff is what's called "general cargo".
Whoever owns them, they aren't poaying to get them back across the ocean. They are only returned if there is export coming back. And in the case of the majority of such containers there isn't enough cargo traffic the return route to get them back. So, they rot or are sold off for pennies on the $
Fine. You win. Shippers are absolutely constantly sending lots of containers to non-container ports where they can't be processed and will forever be out of the system. I don't know where you got the idea I said that no one ever disposed of excess containers. I was explicitly talking about trading with backwaters not designed for container shipping and where there was no chance of those containers going back into circulation.
I also think we need to be careful in ascribing facilities to ports based on their class.
Yes and no. E class ports are absolutely not possessing facilities of any meaningful sort. Class D ports are subject to the usual Traveller ambiguity. They might be a largely abandoned large port, they might reflect a substantial private commercial port with only dregs available to public and non commercial vessels, or they might be just an airport with a water tank to refuel starships. The vast majority of Class D starports have no orbital capacity at all, so they are only designed to handle small ships. Most of them are also in worlds with negative importance, so they are probably not handling very many ships in a given week if you use the WBH or T5 charts for that sort of thing.
Containerized shipping in the commercial/logistics meaning is a major industrial level commercial process. It requires a large transport infrastructure to concentrate commercial goods at a port that has the size and infrastructure to handle big ships and heavy containers. And it requires the vehicles and starships to be designed to optimally handle those standardized containers. That absolutely exists in Traveller in some form, though that that form looks like can be hard to say since surface to orbit part of the flow is not something we get any details about in Traveller and the standard assumption is those large ships don't land at the downport.
However, I have been talking about Free Traders as a class (whether you mean Type A, Type R, Serenity, The Millenium Falcon, or any other such ship) this entire time and discussing cargo handling on that kind of vessel. They are not designed to fit easily into that containerized trade paradigm. They are independent, they do not operate to a schedule, they are too small for any economies of scale, and all the examples we have are horizontal loading, which is not great for big awkward containers.
The primary characteristic that makes a good free trader is being cheap (aka small) and able to go anywhere (streamlined, again small). They need to be able to operate at a class E starport or just land at some winery in the boondocks to bypass the Tukera stranglehold on the port. So it makes no sense for them operating as container ships. Their raison d'etre is going places that probably don't have the facilities to handle them. No matter how disposable you think containers are, no one is sending a containerized ship to the Port of Humboldt Bay. It can't process them.
General Cargo still exists as a shipping class even today because there are places that get trade that can't handle containers and because there are goods that are not ideal for containerization. Free Traders thrive on that. That is what they exist to do. It is why they are designed the way they are.
Traveller is a game. It wants plucky free traders in the stars to be a thing, unlikely as that seems. It tries super super hard to avoid post scarcity despite miniaturized fusion power (Fusion+), unlimited reactionless in system transport, advanced robots, and fabricators. The important thing is to design this imaginary trade system to be fun for adventuring. What that means to your table is going to be different than it is at mine.
I think that anti grav so controllable it's TK is not good for adventuring (because of the implications for PCs boarding or being boarded).
I think that exo-loaders are cool even though they (like all mechs) are actually stupid compared to regular vehicles.
I think that a cargo bay full of all kinds of assorted goods is more fun to have a fight in than one with wall to wall containers.
I think that the PCs being able to land and trade with some outback village is more fun than being limited to a proper container port.
I think the purpose of the trade rules and the free trader designs is to get the PCs into adventures, not to be optimal for corporate efficiency.
If you come in with different assumptions about what is fun you absolutely should make different decisions about how free traders work in your game. That is, in fact, the point of the ambiguity that some people hate so much.