Another facet of your final statement: You also can't measure Firm Specific Knowledge. Especially given how companies drink their own branding kool-aid and refuse to call things universal names (looking at you MicroStrategy, "attribute" is a very doctrinal term for databases, not some random term you put in your BI User Guide), companies often forget to measure the productive effect of understanding the cultural idiosyncrasies of their own company. I think the real big block to eternal contractors everywhere is going to be this plus CEO egos. They will want people who worship the local god of the brand. They will want cronies. We could all rant on this line, or other, actual practical lines of reasoning, like knowing the right people at a firm, office politics, preferred methods of documentation or coding, etc.
I wanted to try and formalize this (how to measure the value of an employee's firm specific knowledge), but my advisor said "Yeah, you and every other PhD." This was in 2018.