Well said. I can tell you're speaking from hard-earned experience. To respect your aging parent's agency as long as their mental faculties permit is one of the greatest acts of love one can give to their parent. It might be hard to watch at times -- a single fall can turn a man who is a young 80 into a man who is an old 80. But our obligation is not to infantilize our aging parents, but to love, respect, and help them when it's needed. They know they're a banana peel away from the old folk's home or a cold slab. They've wrestled with, reasoned with, and made peace with the indignities of old age and infirmity. They know what lies around the next bend, and it ain't pretty. Respect their choices. Bad stuff happens to old people even when they are compliant with the wishes of those who would protect them. Give them their freedom and dignity even though it comes with risk.

I don't say you shouldn't reason with them, plead with them, and explain the dangers to them. But never forget to respect their wishes. They know what comes next and accept it. Do you?

The best thing to do would be to get a motorised chair for the stairs if possible.

Just in case, I mean it's obvious that his Dad doesn't need it, but maybe one day he will ;).

Yes, sounds like this could be a wise move. My dad (now 95, living alone in his 3-storey house since mum died around 7 years ago) had one installed a few years back, and although he doesn't always use it, it's invaluable for the times/days when he's feeling that bit less mobile. Without it, he couldn't have stayed in the house -- and leaving it is almost unthinkable. (Sure, some day there may be no choice. But it has unquestionably given him additional years of independence and stability in his own home.)