But perhaps the target market is changing...Why should that be the case? The resolution expectation should be driven by the needs of the target market. Hint: there’s likely not much real overlap between the target markets for different brands’ flagship cameras.
You could say that recent technology improvements have meant that flagship is now the jack-of-all-trades model that perhaps the mid market was previously. Bandwidth limitations etc has meant that the fastest frame rates were limited to smaller sensor resolution... we aren't really seeing that now. The use cases needing 30 or 40fps vs 20fps is for a small niche.
As you have mentioned, cashed up enthusiasts are probably as big or a larger market for bodies than professional sports/photo journalists in the past.
High mp bodies can now have fast frame rates and and the pricing/features of the Z8 show the market what is possible albeit Nikon has to hit hard to keep/improve their market share.
Yes, users are likely to stay within their OEM's ecosystem but for some people a particular lens/body combination may tempt them to switch.
For businesses looking for tools, they are more likely to get the right combination for their needs irrespective of the OEM and if they can get one body that can do high res and speed then it makes sense to avoid having 2 bodies.