apple designs are mostly about being lean and functional minimalism. this is at every level: from the clean design of open spaces in apple stores, to the look of the hardware too .. laptops, phones, accessories and every things: few buttons, and sleek minimalist design. this also used to be true about the appearance of macos, that took care to look good while avoiding wasting space and also avoided filling ui elements with too much buttons or too big buttons or packing too much stuffs in a given spot. now, big sur new theme is the first step of a departure from that philosophy. wasting space with larger ui element is a shift away from what was a standard at apple, that used to be applied globaly with consistency.
if you look at it, from macos 7,8,9 to macos 10 series it was always the case. nice looking, minimalist ui, each ui element took care to reduce its 'foot print' and avoid space waste and intrusion.
now we see something different that depart from that trend we always had with apple.
my guess is that they lost the main individual responsible for UI direction. in other words: the person that used to decide what the OS was supposed to look like left. now apple might have tried to put a spin on it and have someone new try to fill the vacant position. but unfortunately that person doesn't seem to share the same philosophy principle that made apple products look like apple products. that person doesn't understand the minimalist 'zen' approach. and that person taste regarding colours choices is also not the best to say the least. the result is that apple lost some of its face. some of what made its identity.
it's kind of sad. the end of an era. it's like if suddenly japanese traditional gardens started to be about having lots of flashy colours and modern gizmos everywhere.. that would also be a departure from the usual. from natural and minimalist to flashy and wasteful of space.
it appears to me, the ones saying it's all for the better either didn't understood a thing about apple past designs or are just pretending it is in accordance to their will because they have no other options.
it's the classical case: when people can't have something they desire, often they pretend that it is not good enough anyway and even deny ever having had a desire for it. "in the refusal of admission they've been denied their wishes they pretend they never had such a desire anyway".