When well-known people endorse products, it can be valuable for the seller of the product. But it's also promotion for the endorser, which may be even more valuable than whatever fees and free stuff they get as part of the deal. Their public persona is like a brand in itself, and the more prominently it's displayed, the more valuable it becomes for them in other, future deals. In an extreme version of that scenario, some people worry that some day, an unfit person could even find their way into public office by using the techniques of celebrity salesmanship. (I rather doubt it; people are rational, and aren't swayed by their feelings where something so important is concerned.)
--As for the useful role that a recording engineer could play in capsule design, my experience may be relevant, since I've been a "beta tester" for two capsule types. I recorded concerts with them and gave my observations to the company involved--even sent them samples of my recordings in the more recent case. There was real back-and-forth discussion, and I know that I wasn't the only (nor, frankly, the most prominent) engineer testing these capsules. Neumann has mentioned their version of this process quite openly, e.g. in their origin stories for the TLM 103 via the short-lived model TLM 171, as well as one or another of their modern-day offshoots of the M 50 (I can't remember which one), regarding the way its membrane material was chosen. I would frankly be very surprised if any microphone manufacturer in the world would risk introducing a new capsule or microphone without asking at least one outside recording engineer to try it out first--and before that happens, they will almost certainly have made their own test recordings with it.
But giving our observations and opinions is really all that we sound engineers can do. The actual acoustical and mechanical design of capsules, particularly for directional microphones, is a highly specialized field. To say that "it isn't widely understood" would be an extreme understatement. Even reverse-engineering and copying a known, good-sounding capsule gets you only partway there if you don't know what you're doing in the first place--which very few people do. And they generally keep some parts of that knowledge to themselves, at least in the "old school" way of doing things.
This can create real problems for an established manufacturer (say, in a musical capital of Europe) if there's demand for one of their older designs, but it's been so long that all the people who originally worked on it are gone, and no one who is still in the company knows what made it work, or even exactly how it was assembled. They may have all the old drawings and some leftover, original parts--but in the end they simply become one more imitator of their own original, and not necessarily the most successful at it, either. It's not an enviable position to be in.
--best regards
