Tributaries represent forms of government more common outside of Europe than within. Trading cities represent city state alliances and coalitions.
You can have large or small tributaries, trading cities have to remain OPMs.
Trading cities as subjects because tributaries are subjects and you can do lots to tributaries that you cant do to trading cities makes no logical sense
Trading cities are practically subjects, you seem confused in what they are. We are not talking about cities you invite to trade league, which is city-state alliances and coalitions. We are talking about provinces that merchant republics release from their own territory as satellites that will never leave your trade league and will refuse vassalization even if the conditions are met otherwise. They likely didn't add them as subjects when they were introduced because then only direct subjects with no independent actions were in subjects tab, while tributaries are not direct subjects and still pursue their own politics.
Explain, in simplest terms, why do you think that Paradox made it so city-states won't accept vassalization but will accept becoming tributaries? I am not sure why you are so hellbent on defending a clear oversight that was made because one feature was added after another, much like plenty of other features that went by in long development of this game. If by some chance they added trading cities after they introduced tributaries, they would have made trading cities also not accept becoming tributaries, as how they refuse vassalization. As I said it is common that people argue they should be direct subjects, let alone being like tributaries, while I think they should representatively be your satellites, still able to get into alliances and the like but shouldn't accept becoming tributaries, much like how they don't accept vassalization.
You don't need to defend a trading city if you just keep it encircled in your own land.
I know this, but this is an inelegant and ugly solution. Especially when this is such an easy thing for Paradox to fix but haven't in such a long time. Thankfully they fixed some other issues with trading cities recently, like them acquiring territory and leaving the trade league. Which is why I made this thread as it seems they want to fix the glaring issues regarding trading cities.