Which language shares the highest number of phonemes (sounds)...

>>So, if the alphabet were extended ''bath'' should be spelled with the [@] letter not the [a:] letter because most people pronounce it with the [@] sound. <<

Most people pronounce "bath" /ba:th/ if you include the rest of the English speaking world. (not just those countries you mentioned)

Anyway, why change the spelling over something so trivial that has two standard pronunciations?

More to the point. Why are you changing the topic to something so trivial as spelling reform proposals?