friends told me the expression "wa sai" is out of circulation in taiwan and the in expression for young people nowadays is "wo kau", or simply "kau"...they're getting lazier for sure
but i still occasionally hear people say "wa sai", tho' much less often than before
new expressions come and most would go later. a typical example was "菜" as an adjective, in the 70s and 80s
that's the case in english as well. "gay" has picked on a different meaning. "right on" was as popular in US in 70s as "wa sai" was in taiwan. nobody say "swell" anymore. young people say "cool" instead
same is true for idioms. some are very rarely used nowadays, like "it real sticks to the ribs", meaning having had a very full meal. some are totally out, like "tickling the ivories", meaning playing piano
those that stay become part of the everyday language. i'd guess "帥", "屌", "賊" are such cases