There isn't too much confusion about 'a few bob' although very few young people know that it is/was a slang term for a shilling, and not money in general. On the other hand there is obviously still a lot of confusion about "road tax". OK, "road tax" is a slang term for VED or car tax - the problem is the implication that the 'few bob' collected is spent specifically on roads, and that only those who pay it are entitled to use those roads, which, as we should all know by now, it is not, etc etc etc. Language can and does change, some expressions fall out of use, or become unacceptable.