People seem to be misunderstanding what the OP is stating.
Currently, to change what chat channel, not tab, you are typing in, you have to either click the arrow at the bottom left and select from the dropdown menu, or you have to type in /say or whichever you want.
OP is suggesting all the options be shown below and selectable outside of the dropdown menu.
For the suggestion, I don’t think it would make too much of a difference if implemented. It doesn’t add any new functionality and would clutter the chat window. For the visual prompt of highlighting the buttons, that too is already present in functionality. The current systems has the name of the channel to the left of where you type, and it is color coordinated. The only nice perk I can see would be limiting the options shown below to the channels the tab you have open displays.
I wouldn’t necessarily be against this, but I think the current system is adequate.