Of the big specs in the 90s, it looks like about half of them didn't get made.

churnage, that is one of the pet projects I have on a list: To go through all the spec script sales on the comprehensive list and see how many of them got made, how many by year, what genres did/did not get made, etc. But yes, there was a period of time in the mid-90s especially where a lot of specs were bought, but not produced. In Hollywood, that period is known as a time of 'stupid money.' That era is long gone and it's a testament to the quality of current writing that as many specs sell today as they do in this time of Hollywood's fiscal constraint.