I agree with everythin you said, sea gate castle is insane. Pro players don’t trade because maps like altai, mountain pass or french pass aren’t in tourney map pool. But they do exist in s3 qm pool.
Regardless of that I remember in the pup patch note or in the pup announcement, devs mentioned they’ll further rework the trade to make it more part of the gameplay. Now If they do it, Ottomans would be easily S+ tier for the reasons you mentioned previously. It is insane insane pop effiecency and addition to that they have the most pop effiecent unit in the entire game. Like you could just keep 30 on wood 30 on food and spam jans and gbombards for eternity.
Edit: for giving further math context to why its so good:
Trade bags make traders %40 carry more, sea gate makes tham %40 faster. If you were to use both of those you would have:
X*(14/10)^2=1.96X
X: default trader gold/min
%96 more effiecent traders. And best part is making them super effiecent is only one aspect of this landmark. There’s also armor and mobility like you mentioned.