“If most people aren’t paying attention anyways, so why do you care if their way of not paying attention is by learning.”
The OP mentioned a specific Mishne Brurah. Aside from the fact that this statement ignores the M”B as pointed out by the OP, the balance of the M”B explains why one should not learn. Read his words. The M”B doesnt want anyone spacing out, and certainly not speaking devarim betailim during chazaras hashatz. If they see “chashuva people” not taking it seriously, they wont either. He uses the term machti es harabim.
When my Rav shlita was discussing shnayim mikra recently, he said in the name of R’ Chaim Kanievsky Shlita, that being maaver sidrah during chazaras hasshatz is a mitzvah habaa biaveira. I have no source for this, other than his word.
Just curious. In what sefer is the hetter to blatantly ignore the shatz by opening a sefer to learn found?