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Bike gears wear out pretty fast too, so you'd want to factor that into the equation (different schools of thought on this; some replace the cassette every time the chain is replaced, some replace it every 2-3 chains, some replace chains much much frequently so that they don't have to replace the cassette as often).

I'm curious how often you'd need to replace other moving parts on the stringbike; at a guess since there's much less exposed metal-on-metal there'd be less wear.

You don’t have to change the cassette out frequently at all if you regularly maintain your chain and replace it when the stretch becomes too great.
As said, there are different schools of thought. The smaller cogs in particular will wear out the fastest; if you're using them a lot, the cog wears, which then causes the chain to wear faster, which will then cause the other cogs to wear faster.
It's sort of the opposite, a worn chain will wear on your cassette. Using a chain checker, or a ruler, you can measure how worn your chain is. If you replace the chain too late, the new chain will not mesh with the cassette, and it will slip. You can usually do this 3-4 times before the cassette will also need to be replaced. How long you will be able to use a chain is dependent on the riding environment, chain material, how well it is cleaned, and how well it is lubricated.

With an 8 speed bike, riding between 300km - 500km a week, I usually get around ~1000km on a chain riding outdoors, or about a month on a chain. Riding indoors on a trainer however, my chain almost never needs to be replaced.

For an 8 speed bike you replace the chain at 0.75 wear.

Depending on the bike you may choose to let both wear, and replace both at the same time. Eventually you will find the shifting sloppy, and the chain slipping however. This observation is from personal experience.

I find the idea of using string interesting, more commonly on the market you would see belt driven designs like the pinion.

The maths:

500km == 310.68m

1,500km == 932.05m

500m == 804.67km

5,000m == 8046.72km

I got confused a second.

you should write "mi" not "m", m symbol for meter.

It might cost more to manufacture in the USA, but that 61% increase to minimum longevity sure would be nice.
What is this comment in response to?
It appears to be mild snark about the mixed units (km vs miles) caused by quoting two different sources.
Minimum time to failure seems to be 500 in either unit, so it makes sense to move manufacturing (or at least testing) the the region that uses the larger unit.
Ok. Silly quips aside, bike chains do actually last 5000 miles, not 500, if you keep them relatively clean. That's a lot better than 1500 miles or kilometers.
If this rope works well and is easy to replace, and is inexpensive I think I would be fine with changing it every 300 to 1,000 miles or so. They have it listed that you can change colors to match your outfit, so I am expecting it to be that easy.

> Change the strings quickly and easily in 2 minutes, no tool required and no need to remove the wheel. Choose the color to match your outfit or mood.

They say a few minutes, but with a lockring tool and a chain with a masterlink, I bet someone with a bit of experience could change a cassette in under 2 minutes as well. The rest of the mechanism seems a lot less durable and a lot less amenable to off-road riding where one might wish to pedal backward up to a full revolution to reposition their feet without wishing to propel the bike forward. It might be OK for casual / commuter bikes, but belt drives already exist and last a long time. The width of the rear axle required for the dual-sided drive also looks super goofy and probably gives a poor Q factor ( https://www.cyclingnews.com/features/what-is-q-factor-and-do... ). Overall these seem like a lot of compromises for little benefit beyond a chain or belt. Chain-driven bicycles are also incredibly robust and proven to work amazingly well in a very wide range of conditions, and are serviceable worldwide.
That's a simplistic analysis. Quality chains almost certainly have a life longer than 500 miles, even taking into account premature failures.

Regardless, aren't you suggesting here that we should use chains and not strings because their upper bound of lifetime is higher?

My Gates belt probably has >10k miles on it and is also silent. Coupled with a Rohloff, it is an essentially maintenance free drivetrain, save for changing the hub oil once per year.
Too bad that the Rohloff setup alone is like $1600, and the non-electric bikes with them tend to be >$5000. That Santos Cross Lite with the Lauf suspension fork does look pretty trick though. They have a helpful bike finder, though lots are not available in the US https://www.rohloff.de/en/service/search/bike-finder