I think standard zoom lenses are the toughest category of them all in terms of design.
The Canon 24-105mm L IS II already showed that 50mp are a borderline thing. In comparison the Sony GM did great.
Still - pushing megapixels much further just doesn't make much sense in my opinion.
I think that what is happening is that we are finally being able to see the real performance of lenses with these high MPs. In a way it is throwback to the analog area 30 or so years ago.
MTF curves look like what they were with film again, markedly higher in the center than at the edges, especially at the larger apertures.
I reckon due to the way sensors, sensor structure and digital sampling work, with sensors up to, say, 30 MP there is an artficial ceiling to resolution, so that everything over the entire frame gets equalized to a degree. With higher resolution sensors we now see the limits in the corners and off-center areas.
The maximum center resolution we haven't reached yet, I think, considering how some lenses score at the center. So a few more MPs would help there
Kind regards, Wim