QueenAlice said: "Yes, once upon a time it was common for a show to drastically change during previews. This happens less for several reasons, the principal reason being that the technical elements of shows today (automation in particular) make it much more difficult to make changes that don't also require a major 're-tech" of major production elements. Such a re-tech is hugely time consuming and expensive. Ah for the old days when tech elements consisted of four lighting cues and a series of back drop curtains!
Secondly, and this is my own opinion, I think creative teams today largely don't have the tool belt to know how to fix shows during previews. The skill to write a musical is one set - the skill to re-write or actually improve it is another. A constant you will find on these boards is bewilderment about how little a show is often changed either in previews or from an out of town engagement to a Broadway opening despite the need for changes seemingly being in clear view."
A major reason that you're forgetting is that shows spend far more time in development nowadays than they did back in the day. Shows often would go into the rehearsal period for the Broadway production without ever having workshopped it, and that means they would only have 6-8 weeks from first rehearsal to final preview to get the show in the shape they want it to be in. And many would also have out-of-town tryouts, but that would only add another 8 weeks of development. Most shows now are workshopped for years and years. Just look at Hadestown's journey, starting as a concept album in 2010 and having three major runs of the show before making it to Broadway 9 years later. Come From Away had a very similar process, starting as a college workshop in 2012 and having four major regional runs before hitting Broadway in 2017