Mizuno's market share in the tech run category has dropped consistently over the past couple of quarters according to Leisure Trends.
When I used to work on the retail side of things (4 or so years ago) you couldn't get someone to change out of their Mizuno's. There was a very particular foot type that worked with mizuno, and if you didn't have it, they sucked. Very narrow, low volume foot. So when someone would come into the shop looking for new trainers, we didn't even bother trying to get them in anything different. Then Mizuno started beefing up their shoes. More volume to the upper, wider midfoot, and taller midsoles. This alienated their core customer in an attempt to "American-ize" their shoes and make them more appealing to the general public. Like I said, this alienated their core customers and then threw them into the general noise on the wall where they couldn't stand out.
But hey, what would I know.