” Only Talmidei Chachamim whose entire existence is toiling in Torah, are qualified to establish new minhagim, or to change existing ones. “
Sometimes new minhagim arise as the result of Talmidei Chachamim, but it is likely that often minhagim do not have any such obvious source.
Examples:
Which Talmid Chacham instituted Hallel on Rosh Chodesh?
Which Talmid Chacham instituted Simchat Torah for the second day of Shemini Atzeret in galut?
Which Talmid Chacham instituted not eating kitniyot on Pesach?
Which Talmid Chacham added corn (maize) to the minhag of not eating kitniyot?
Which Talmid Chacham instituted men not wearing a talit gadol prior to marriage?
“whose entire existence is toiling in Torah”
Many Talmidei Chachamim are well learned in other areas as well, baruch HaShem.