There have actually been two big budget productions of Marilyn. In addition to the debacle at the Minskoff, there was a totally different production around the same time in London. It was much better than the Broadway one (light years), but too derivative of Evita, with a character called 'the camera' substituting for Che, and both actively participating and commenting on the action around it. IT was well done, but its lack or originality killed it at a minimum.
I think the idea of minimizing Smash is much better than Bombshell. We only saw snippets of Bombshell (including the three different versions of Act 2 opening, two of which would have to go), but we got to like many of the characters putting on the show. Clearly, with a 2 1/2 hour musical, they will need to limit scope, but that can be done.
To me the problem with any musical of Marilyn is deciding what story to tell, how many key characters to include, etc. The original Broadway version had so many 'commenters' that there was nothing but Marilyn and uninteresting duos or trios (e.g., (as I remember), a group of three fans with no individual identity, two gossip columnists with no individual identity, even the three husbands had virtually no real identity, as I recall, but I saw it once 30 plus years ago...).