I wonder, would agricultural labour be backbreacking if you only had to meet your own demand (and owned enough fields for the purpose)? Serious question, I don't know.

I've only had to work in vegetable and fruit gardens belonging to my grandmother, but if it scales, then yes, it would be backbreaking. Only worth it if you enjoy it or otherwise derive self-actualization from it etc.

Yes it is backbreaking or you are trapped by low margins into ever increasing size of fields so you can afford technology not to break your back.

Nature is too erratic to not have episodes of famine if relying only on sustenance farming.

1) There honestly are people who enjoy this kind of employment.

2) More usually, manufacturing work is appealing because it can be an easy springboard to a more rewarding role/career. It's still true today that thousands and thousands of Americans who have started (or are starting) their career as an assembler or machine operator have had the drive to improve their skills & knowledge and move far upward in the value chain.

1) I would enjoy doing this kind of work for 30 minutes, in a big factory, with cool heavy machinery. I'm sure I can find a lot of other people who would enjoy it, too. But you really think there are people who would want to "spend the rest of their life doing repetitive assembly" or that it would be good for them?

2) Why is clerical work any less appealing or rewarding in terms of being able to market oneself?

People want to do them when they pay 70k per year with excellent health benefits and excellent pensions. People want it to be like it was when the auto industry was booming. Unforunately, that just doesn't make sense.