This just isn't true in my experience. A really good experienced generalist can make a huge impact, and many places recognize this.
Totally agree on the huge impact, et al. And for available jobs without a job posting, this remains as you say. But when a hiring manager goes to HR/recruiting and a job posting is generated, it is never for a generalist.
So, an experienced generalist has to skip over specific postings, simply look at who is hiring at all (or growing headcount as evidenced by other data points), and find a way in to pitch themselves.