The PAPER it’s printed on might be flexible.
The ideas, rights and principles enshrined in the Constitution itself, are not. They are timeless. They are as applicable today as they were 230+ years ago.
The same ‘changing, flexible’ argument is used against the bible, by pretty much the same people. In both cases they don’t like what the bible/Constitution says, so they invent things they say it really says, to soothe their itchy ears.
There.
Fixed it for 'em.