I think you misread the meaning--I think he's saying "Hey, I don't ever want to work for/work at/start another company". That's the opposite of what you're worried about.
Perhaps he thinks the cofounder wants to coast along in a cushy position for the rest of his life, burdening the company. He's saying "phew! I don't ever have to apply for another job! puts feet on desk"
Not that I agree with it, but I understand the interpretation. It's a fairly common view in the tech industry that you should get a new job every 18 months or else your skills will degrade.
In what way does a co-founder intending to stay with the company put its long-term commitment to paying customers at risk?
My guess is that the statement
"I’d be happy if 37signals is the last place I work."
is very similar to:
"37signals is the last place I'd want to work"
...which is quite a polar difference from the first statement.
It's crazy to me that a co-founder of a company would talk this way about their own business ... at least publicly.
For a company with paying customers, this kind of statement doesn't instill confidence in the long-term commitment of it's founders in the business.