Obviously, the worth via interest can't grow forever, because there is no infinite value going around.
Also, isn't it a good thing if labor becomes less valuable, implying that less labor is REQUIRED? That is what people aim for when they invent technology to make life easier, after all.
I think you may have this backwards.
If I generate $1 of value/hr that's kind of the maximum you can justify my wages being. If you give me a machine that increases my productivity 10 fold and the machine costs $2/hr then my maximum wage tops out at $8.
Now in this situation less labour is required, but the labour isn't less valuable. Obviously now you need a new industry to come along to employ the other 9 people who aren't now needed, possibly now servicing me and my newfound wealth, or possibly building the machines that put them out of a job.
'value' is what someone is willing to pay. My employer would have been willing to pay me a maximum of $1 and $8 pre and post machine introduction, so you could say my value increased. But I'm not likely to see much of that in the short term, because all my mates are now unemployed, and would be willing to do the job for their old pay, so my leverage is very low, so worst case I keep the same pay, in which the value of the job to me hasn't changed.
Of course, as you say, it could work the other way, and working could become more valuable because one worker could generate more returns. But that requires there to be more demand for everything.
Like, say, people eat only potatoes, and all work 8 hours per day to harvest just enough potatoes for everyone. If a machine could do their work, and they couldn't think of anything else to do, there would still be the same amount of potatoes, but less work to do.
Could your pay go down in the example? Theoretically yes, in practice we see the opposite. Economic development couldn't happen if it did. Although I suppose short term dislocations could lead to lower pay in the short term, before increasing long term?
Don't forget 'progress' is people increasing productivity. In a world where everyone has to work full time digging potatoes, that's all anyone has. If it only takes 1 member of the group to dig potatoes, that frees the other members to discover science, religion, chips. We are in a situation now where a fraction of the population work in food production, but we all have jobs and more wealth than potato growing us could possibly imagine.
That added productivity doesn't disappear, in our earlier example the owner of the means of production has captured all the value that I didn't get from it. He has to do something with it. Even if he puts it in the bank, that's possible seed money for a venture that ends up employing my 9 ex workmates.
Also don't forget that the one potato digger won't want to dig potatoes all day whilst everyone else sits around. If there's no reason to dig more he'll dig enough for himself and take the rest of the day off. Now you could entice him to dig for you in exchange for beans, because you know he doesn't like picking beans, and as you don't like digging potatoes. And of course I could offer to trade apples...
The problem is that life is not easier because all that efficiency is captured by a tiny group of non-laborers.
The people whose labor was replaced are not benefiting from the end of their labor. They're now jobless and forced to compete harder for lower pay.
But sure, there are various ways of looking at it. As others have pointed out, with increased productivity, the value of labor could also increase.
But it is of course a relevant question how to measure.
I suppose you can also get a computer that is as expensive or more expensive than a computer 20 years ago. But it would also be a million times faster (or more, I am too lazy to look). You could also get a Raspberry Pie for 30$.
I used to think this.
I came to the conclusion that families and familial lines, like individual people, are living organisms.
Some of those living organisms (familial lines) are relatively young and inexperienced - and they will not gain that experience within one family members' lifetime. It will take multiple generations of growth and nurturing to "grow up" to produce individuals, like you, who can operate in a more complicated society and transact more abstracted work products for more abstracted rewards.
In the meantime, as in all of history, members of juvenile family lines need meaningful, satisfying labor that they can transact for reasonable currency to thrive as a living organism.
Asking why we need any labor to be done is like asking why all those infants can't just get with the program and eat solid foods already.
If less labor is required for tasks A, B and C, then that frees up people to do new tasks D, E and F.
It is untrue that there is a fixed amount of work to be done, so when some of it is automated, then that necessarily creates unemployment. That's the "lump of labor fallacy".
https://en.wikipedia.org/wiki/Lump_of_labour_fallacy
It's also fallacious thinking to assume that every person must be employed working for wages.
If someday the economy as a whole can produce everything people need by employing 10% of the people to build robots, then we can organize it such that the people who like to build robots can do that, and the people who like to paint, or sculpt, or hike or sail can do that.
There is no universal law of the universe that requires the 10% of robot builders to own everything on earth, and the other 90% should live in rags, filth and sickness.
In your universe the universal law is that the robot builders have (collectively) nigh absolute leverage over the rest of society. Without the angel assumption they would slowly (or quickly) use that leverage to amass power.
People wouldn't live in rags or whatever but all secondary economies point to providing value to the robot builders.
But wages are always relative to prices. If everything is being made by robots that nobody has a monopoly on then everything should be very inexpensive. Having to work for the same nominal wages as you had in 2003 is not that much of a problem if nominal prices are now 5% of what they were then.
The problem comes when you have some kind of corruption or distortion or regulatory compliance costs keeping some prices high (see: housing, education, medicine) even as general wages are flat or decline, or just grow less than those costs.
I know this sounds hard to believe for some. But today, agriculture takes up about 2% of labor, and manufacturing about 10%. And we consume about 2x what we did per person compared to the late 1970's. So, cut back manufacturing to about 4% of the labor force and use another 3% of the labor force for distribution. Voila, 10% of the labor force is used to sustain a 1970's level standard of living. Everyone else can improve their themselves, create arts, study nature or contemplate the nature of existence (that's sarcasm there).