I am not a trademark lawyer. I'm guessing you aren't either or you would have said so. But multiple lawyers on multiple projects with multiple brands have mentioned this to me, and we have sent C&Ds to protect trademarks. With at least two of those lawyers, sure, it might just be more billable hours. But two were pretty decent guys and wouldn't have done it if they didn't think it was necessary.

At least within my corner of the business world (and not just "the internet"), it seems this is common knowledge among lawyers.

My guess is that, sure, would a court throw out a trademark? Maybe not, but the law works on a fine line between actuals (precendent) and hypotheticals, and it's just cheaper to issue a C&D than it is to fight a prologned legal battle about predcedent.

Also, it's interesting you mention Frisbee. There's a term for that: "genericide," and it's the term used to describe that exact scenario. Frisbee is one brand that didn't actively protect against trademark erosion and now we call every flying disc a Frisbee. Retrospectively, Frisbee might have wished they'd sent more C&Ds. Nintendo is one brand that has actively protected brand erosion, which is why we don't call it a Sony Nintendo.