FCP7 should easily install on El Capitan. Remember to first install QuickTime 7.6.
Concerning the "reprieve" for those people who have given up their pre-High-Sierra systems. Lauren's suggestion to use Retroactive for installing FCP7 on some later systems simply to export .xml's from FCP7 could be useful. However, FCP7 installed using Retroactive is not fully capable FCP7. I was surprised to find that Retroactive did not enable installation of Compressor 3.5 from the same Final Cut Studio package. FCP7 interacts with Compressor 3.5, e.g., when using FCP7 "share" to make Blu-rays.
Dennis Couzin
Berlin, Germany