This is so subjective and dependent on use cases. But it's a well know thing, psycologically, that people have a hard time understanding others when they don't do things the same way they do.
For my, subjective, use case I need both sizes, and while sure technically I could give up the advantages of each for the other, I wouldn't want to.
For me it's simple.
At home I only use the 12.9 as it's better for everything. I don't hold it for long other than sometimes for reading Word or PDF documents and for that I use my 2018 one (lighter) instead of my M1 (heavier), so yes I have more than one 12.9... Other than that I have Magic keyboards in my home office and a stand in every other room. The only time the 12.9 goes out of my home is to go to my holiday home....
For on the go I use the 11in and the 10.5in, mainly because they are more compact and easily fit everywhere (and they are always in keyboard cases on the go to protect them, while the 12.9 are mainly used naked at home, which offsets the additional weight).