"They never pay the slaves enough so they can get free, just enough so they can stay alive and come back to work. I could see all this. Why couldn't they? I figured the park bench was just as good or being a barfly was just as good"
Could have been posted on Reddit IAmVerySmart or im14andthisisdeep for the style of writing. He genuinely believes that he is more clever and seeing something they cannot see? (Why does he think they are unhappy at all if they can't see that?). And is somehow working under the idea that he could have no income and a bar would still serve him alcohol, because if it's his role in life to be the more-intelligent-than-you outsider, of course the bar would fall in line with this vision (narcissism). And note that after seeing this truth which they were too dumb to see .. he didn't go and live on a park bench. It clearly was not "just as good" to him, he only wants to posture that it was as good so he looks better. So much for deep insight nobody else possessed, he was doing the exact same thing they were.
They know this. Everyone knows this. Everyone who has had a job they don't like for more than a month realises they have no way out. He even quotes several of them saying that exact thing he claims they can't see, in his letter.
I now write from an old mind and an old body, long beyond the time when most men would ever think of continuing such a thing,
Oh fck right off, 66 at the time of writing and still singing his own praises like that? The USA and UK are raising state pension age to 67 as we speak, and that's for labouring jobs as well. Working at age 66, Writing at age 66, is no superior feat of endurance. Admiral Grace Hopper retired at age 60, was recalled to the Navy within a year, retired again four years later. Returned to active duty again, was promoted again, retired again at age 79, then went to be a senior consultant at DEC and stayed employed there until her death age 85. And she wasn't boasting about how great she was, she was talking about the younger people she was training and supporting.
Even Heinlein doesn't explicitly self-promote this hard, at least he tries to lecture the reader with "worldly wisdom" for the reader's own benefit, and you just get the superiority as a secondary effect from the way every wise character is Mary-Sue, and it's never done in a "plus you're a loser" way.
He is celebrating his freedom from bondage at age 65, from the tyranny of a boss wielding his fate at a whim
According to the article his income depended on him being a full-time author, and the whim of a wealthy patron. And his "freedom" was age 51.
from suffering the fate of “emptying out”
Jobs he somehow survived thirty years of, and didn't quit to be on a park bench, and didn't consider himself empty and broken and devoid of anything to write about. I guess that's just another way he's better than all those other shlubs, eh?