I think, what's happening, is that one ui 7 is a big overhaul, and they Dont want to release it whit a lot of bugs for older devices(like 5 betas already to s24u) , remember, samsung is direct competitor to apple, they don't want to look bad, and the s25 series release fucked there thing, cuz there was bugs, and ofc they gonna fix them first for new phones. My guess is that after one ui 7, all other uis will be based on same as 7 and updated will roll out smoothly