Here's from the ep. "Boo" in season 11, dialog between F & N after Martin's heart attack:

Frasier: You know, eleven years ago when he moved in here, I couldn't
imagine a bigger infringement on my life. Now, I can't imagine my
life without him.
Niles: It would be very hard to walk into this apartment and see
that chair and know that Dad wasn't here anymore.
Frasier: Oh, I'd get rid of that chair immediately.

I think if anything happened to Martin in the series, in reality of the show, I think the last thing Frasier would get rid of is the chair. What do you think?

Absolutely. He'd have the same thoughts and memories about that chair that Martin tried to explain way back in season one when Frasier didn't want him to bring the chair to begin with, and when Frasier tried to buy him a new chair and then had to go to the school where they were using it in a play.

Obviously the line was comic relief, but Niles knew it too, and so did Frasier. Sweet. :)

I bet he'd keep it too. I mean, every time he saw it, he'd think of Martin and somehow that woudl keep him with them a little longer, in spirit anyway.