The greatest reason I believe a company should prioritize for cultural excellence is for its causation effect on product excellence. I firmly believe this causation exists and this mutually exclusive scenario you mention doesn't exist.
My points are:
a) it seems that Meetup tried to optimise their culture for aspects that were not directly related to --and controversially, maybe even opposed to-- product excellence. i.e. their initial version of 'cultural excellence' was more about creating a pleasant inclusive place to work, rather than a productive or high-performing place to work.
b) further, if (as a result) your hiring decisions are more guided by prioritising people that will fit into and perpetuate that cultural vision, you're implicitly choosing to not prioritise hiring high-performers.
(As said before, it's not mutually exclusive - there may be some high-performers that do fit into your cultural vision... but on a macro scale, if you follow that particular cultural vision, the trend will be away from excellence and performance.)
I think I read Paul Graham (or someone from YC) stating that you want diversity of background on your team, but not diversity of objectives. I wholeheartedly agree with this. The more diversity on your team (regarding every dimension: gender, race, nationality, personality traits, education, etc), the more chance you have at building a great product; as long as all of them have unity of vision and goals.
As an anecdotal example, I was hired as a software developer on a team of mostly electrical engineers, and our group develops software/supports the energy management system at a major utility company. Now when I was interviewing, what the team seemed to like the most was my background in actual electric utility operations (I was in the electrical trades for a long time before I figured out that I wanted to study CS). I was the only person on the team with actual operations experience.
My boss love it because I could go and talk with the operators, and speak "operator" with them. It made translating their requests into features/upgrades/updates a lot easier for our team, and our customer service improved as a result. By having that background in operations, which was different from the rest of my team, I was able to bring a different perspective, and most importantly, the perspective of our customers to our day to day work.
You could argue that the reason why Meetup didn't have a culture of innovation was because leadership was too busy optimizing for an inclusive culture. But again, an innovative culture and an inclusive one are not mutually exclusive, and the article doesn't support the conclusion that Meetup's culture was getting in the way of innovation.
In fact, the employee you quote and most of the article goes against your second point. It seems that their previous culture was the only advantage Meetup had going for it when it came to recruiting. They were able to hire people of much higher caliber than the product warranted or deserved because of that culture. And once they changed that culture they began to lose talent.
[ADDED: I agree with your sibling comment. So long as a business can continue to operate in a manner satisfactory to the owners/management, a variety of different cultures and management styles are perhaps equally valid.]
Such a company can only exist in a fantasy land where growth and expansion are always eschewed for social justice.
That's just as bad as rudderlessness, if not worse. The answer needn't be an extreme, and these discussions needn't consider business success as the sole axis of correctness--ethics should be considered as well, whether or not it has a monetary value.
B) your assertion seems to be disproven by the world. Look at the top tech companies with the best products; they are all embroiled in drama with regards to toxic culture one way or another. The market does not seem to select for workplace culture.
If what you say were true, optimising for diversity would be totally irrelevant. I don't know if any company that is more racist than it is profit seeking.