How can one respond to someone who says, "The important thing is that it's not forbidden," when advised that some matters are innovations (bid'ah)?

The origin of the error is the assumption that innovation (bid'ah) is not forbidden, whereas it is indeed forbidden, according to the saying of the Prophet, peace be upon him: "Every innovation is misguidance." And among the misfortunes of innovation is that its perpetrator is veiled from repentance until he abandons it, as the Prophet, peace be upon him, said: "Indeed, Allah has withheld repentance from every innovator until he abandons his innovation."