Guy I knew went to one of their Farewell Tour concerts in the mid-90s. His response when I asked him how it was:
"When I was in high school, they were the #1 cool band that everyone wanted to imitate. So I got used to the sound of a bad band trying to play Rolling Stones songs.
I’d almost forgotten what that sounds like"
I know it’s an old joke/trope that the Stones have had farewell tours for the last 40 years, but can anybody produce a cite from one of the crucial band members that declared any special tour their last or farewell? Because I don’t remember any. Speculation by the press and fans, sure, loads of, but nothing official.
The 1979 film Americathon thought it would be funny if an old band like the Beach Boys was still around in 1998.
Maybe this Keith interview from Guitar World? For context, he’d just explained that due to Charlie’s cancer they began working up songs for A Bigger Bang with Mick on drums:
GW If the day ever came—God forbid—that Charlie Watts couldn’t make it, would the Stones go on?
RICHARDS That’s a good ’un. Probably not. As you say, God forbid. But at the same time, Mick and I kind of got over that hurdle this time, and said, “Well, we could still make records…”
That might be the quote I sorta think I remember. I doubt that I would have been reading Guitar World, but someone could have quoted it somewhere else.
My sister saw their very first concert in the US - Swing Auditorium in San Bernardino.
They’ve lasted longer than she did. 
I don’t imagine anyone in 1979 expected The Beach Boys to ever be relevant again, but in 1988 there they were with “Kokomo.” And a lot of acts keep touring for decades, two different versions of Herman’s Hermits, one with Peter Noone and the other with Barry Whitwam, are still in business. Why is it so surprising? Folks like Frank Sinatra continued to perform into his 80s. I guess people saw rock & roll as a young person’s game.
I do remember that. The rainy Halloween in Dallas. And when we saw the Stones on the 1st, Mick said something to the audience like, “Did anyone order a fucking ark?”
I mean, they’re musicians - performing is what they do for a living. I’m not going to resent someone for doing their job. If they still want to work, and people still want to pay them, then why not?
I’m not going to resent them either. A lot of people have some skewed ideas of what it means to be successful in music or other art I think. They see a group like the Beach Boys who were so huge in the 60s, and kind of laugh at them still going at it in the 80s when they didn’t seem at all relevant. But the truth is, if you’re making a living performing music you’re a success even if you’re not filling stadiums up like you did in previous decades.
For a lot of artists, I think the need to perform isn’t based off of economic need. Phil Collins probably doesn’t need to make more money, but he came out of retirement in 2015 to continue perform. Though some, like Leonard Cohen and Chuck Berry really needed the money in their later days.
In The Wrestler, Micky Rourke plays an aging pro-wrestler whose heyday is long behind him. In the 80s, he wrestled in front of an audience of tens of thousands, he starred in his own Nintendo game, and one woman he meets mentioned that her brother had a poster of him on the wall. Now he’s living in a trailer park, has to hold a job down at a grocery store to make ends meet, and performs in front of crowds of hundreds if he’s lucky. There’ a scene where he speaks to his diminished audience and says something like, “They say I should stop doing this. [Pointing to the crowd] But as long as you’re here, I’ll be here.” I think a lot of artists have a similar outlook.