It might seem like domination but its just the biggest field. The pie is huge, everybody needs CMS. So everybody eats and if you focus on one segment it might see like its domination.
I would say if anything every other website now is Webflow. Every shop is Shopify. And highest quality CMSes are commercial products like Craft CMS or Kirby CMS. So serious projects often grab those instead of OSS offerings.
Ok, there is for sure a closed source offering. Although, if you look at the data of the 1 millio top site on builtwith a strong oss share is clearly visible. It goes without saying that wordpress has the overall largest share, which technically might not be considered a fully-fledged CMS...
I would say if anything every other website now is Webflow. Every shop is Shopify. And highest quality CMSes are commercial products like Craft CMS or Kirby CMS. So serious projects often grab those instead of OSS offerings.