How can one respond to those who believe that praying on a chair is an innovated practice, citing that the Messenger (peace be upon him) did not do so despite the existence of chairs, and that there is no reported tradition from the predecessors indicating its permissibility, and that the Companions used to lean on staffs during Tarawih instead of chairs?

There is no harm in praying on a chair for someone who is unable to stand, due to the Prophet's (peace be upon him) saying to Imran ibn Husayn: "Pray standing; if you are unable, then sitting." Sitting includes sitting on a chair, and it has been established that the Prophet (peace be upon him) and the Companions sat on chairs and various means of transport. Praying on a chair is not an innovation, and the Companions' reliance on staffs was out of their eagerness to attain the full reward of prayer.