I don't understand this point. Why would there be a limit to productivity, especially with machines and computers to leverage our efforts thousandfold and more?
And how could anyone know that such limit exists, even if it is actually reached one day? That would require predicting any possible innovation of the future.
Do you agree that that is a pretty hard limit?
PS: 1% growth per year seems slow and sustainable but in 20,000 years your talking about 2.7 with 86 zeros after it. Start with 1 atom for one second and compare it with every atom in the earth for a year and your only at 4.2 with 57 zeros. So if the total economic output of humanity was 1 atom we could not sustain 1% economic growth per year for 30,000 years.
Edit: This assumes that the economy is based on physical goods, if we assume intangibles can take up 99.99...9% of the economy then 1% growth is possible for long periods of time even if it's somewhat meaningless.
There are limits to what you can do with matter. It's unlikely the economy (our productive capacity) will continue to grow much for 100,000 years after all mass we can get at is weird ass artificial quark composite computronium.
However, energy needs do seem to increase with productivity, and there's limit on how much energy we can get from the sun (a dyson sphere encloses the sun, capturing all its energy). But there's some assumptions here: that we can't discover ways of being productive that require less energy (and wouldn't that enhancement itself be "productive"?); that we can't discover other sources of energy (e.g. room temperature fusion micro-suns - or something better).
Once you factor in the unknown (i.e. knowledge that we don't yet have) it's hard to know what the limits are. I guess the entire universe is a limit on matter... if we can't discover a way to make more matter... or more universes.
We may be quite sure that among plants, as well as among animals, there is a limit to improvement, though we do not exactly know where it is. It is probable that the gardeners who contend for flower prizes have often applied stronger dressing without success. At the same time, it would be highly presumptuous in any man to say, that he had seen the finest carnation or anemone that could ever be made to grow. He might however assert without the smallest chance of being contradicted by a future fact, that no carnation or anemone could ever by cultivation be increased to the size of a large cabbage; and yet there are assignable quantities much greater than a cabbage. No man can say that he has seen the largest ear of wheat, or the largest oak that could ever grow; but he might easily, and with perfect certainty, name a point of magnitude, at which they would not arrive. In all these cases therefore, a careful distinction should be made, between an unlimited progress, and a progress where the limit is merely undefined.
He then observed the tendency of humans to reproduce until the limits of their environment are reached and concluded that poverty will be the terminal state of humanity.
Malthus was merely wrong on this empirical point - humans are willing to voluntarily stop reproducing under certain cultural and economic circumstances. These circumstances involve modern levels of economic productivity, technologies (birth control) invented 100 years after his death, and western cultures. He wasn't wrong on principle, he merely lacked the data we have today.