Unfortunately by definition a show in previews is not "ready". The whole purpose of previews is to try out a show with an audience to assist with its development, especially with brand new shows. Re-writes and constant changes are usually done during previews (for example, Sondheim wrote "Send In The Clowns" while "A Little Night Music" was previewing on Broadway.)
But I agree that theatres should not charge full prices during previews. At one time they didn't do so - but, as with many other aspects of theatre in more recent years, commercial greed has taken over.
THEATRE 2020: CURTAINS**** LET'S HEAR IT FOR THE GIRLS***** WICKED***** KEITH RAMSAY TAKING NOTES WITH EDWARD SECKERSON***** KAYLEIGH MCKNIGHT CONCERT***** RAGS***** ON MCQUILLAN'S HILL** DEAR EVAN HANSEN***** THE JURY***