In anand Sahib, the 19th and 20th Pauris are a very good explanation for the physical rehats.
The first one says that what a Sikh should not be is dirty on the inside and clean on the outside.
The next one says that a Sikh should be clean on the inside and clean on the outside.
The option of being clean on the inside and dirty on the outside is not given.
So when people say that the outside does not matter, they are proven wrong by Guru Amardas Jee. The outside matters just as much as the inside.
These two pauris clearly prove the necessity of keeping kes, dastar and the rest of the Bana. And now people don't have any reason to go up to Khalse that dress in Bana for Guru Jee and tell them to wear "normal clothes," or to not wear a khanda or that big kara. Because the outside matters.