The engineers say that it's an engineering problem. If there's only 1000 tons of helium available, then when we run out of those 1000 tons, we're out of helium.
The historians say that it may seem like an engineering problem, but somehow we never run out of anything.
The economists say that it's 2 or 3 different problems, none of which are the engineering problem that the engineers talk about:
Question 1: Business problem: How valuable is the helium for use #1 vs. use #2. As we start to run out of helium, the price goes up, and childrens helium filled baloons start to cost $5 or $10 or $20 each, instead of $1. All of a sudden, kids don't get baloons for their birthdays...they get stuffed animals instead. And the rate of use of helium drops. If helium costs $1000/baloon-worth...an awful lot of folks will opt not to use helium
Question 1b: What are the best substitutes? In the case of balloons, perhaps a mixture of hydrogen with argon is almost as good as helium for both non-combustibility and lightness (I don't know). Perhaps balloons need to be mounted with non-combustion safety devices at a cost of $6/each in order to make hydrogen safe enough to use instead of helium. If helium costs $10/baloon...welcome safe hydrogen balloons (safe already, hydrogen burns upwards, but whatever).
Question 2: Is it recoverable? Right now most helium baloons are made from semi-permeable rubber-ish stuff, which leads to the baloons slowly leaking. However, a substantial number of baloons, apparently increasing, are made of metallized paper...which means that the helium doesn't escape. Perhaps loaning a group of metallic helium balloons to a kid's party is now doable...with a substantial deposit. Perhaps helium is becoming so scarce that helium recovery devices mounted on airplanes are cost-feasible.
Question 3: Is it produceable/renewable? At what cost of helium does commercial fusion become worth investing in purely, or additionally for the production of helium? Why are there more forests in the US now than there were at the turn of the 20th century? Because we needed wood, and wood grows.
Between fission and fusion, we can also produce any element that is important enough.
Question 4: Can we use less of it to get the same result? It used to take a lot more metal to build a car than it does now. Knowledge is replacing mass rather rapidly. Ditto roughly everything. Turns out that our intuition that manufactured goods are made out of stuff is a mostly wrong, industrial age model. Manufactured goods are mostly made out of knowledge. And the less knowledge you have, the more stuff it takes.
Question 5: What incentives are in place to convince people to push the borders of questions 2-4? Really...that's question #1 again. $100 barrels of oil opened up the North Dakota Shale Oil fields. Shale Gas has deeply transformed the energy industry. What is worth solving? Well...the $ available make a difference on that problem.
But the engineers reply...But surely there is some limit?
And the economist-engineers say...we're not talking quantity really...we're talking rate. Suppose we could build fully recyclable cars, produceable in 15 minutes, google-driven, and uber-safe...provided you had access to 500 pounds of carbon. It could well be that we could build a car to drive to work...drive home, recycle the car into a outdoor bungalow for your party that evening...and then recycle it back into a car for the next day's drive.
And the engineers reply...But surely there is some limit on rate?
Sure...there are two real limits on what we can support. Mass and energy.
The earth's crust masses 10^25 g. The earth masses 6 x 10^27 g. We can't beat that without getting offplanet. The solar system's solids, minus the sun, masses on the order of 10^30g. We can't beat that without getting outside our solar system.
The earth receives on the order of 10^17 watts. We can't beat that without getting offplanet. The sun produces on the order of 10^26 watts. We can't beat that without getting out of the solar system.
So...yes..we know that human beings using beyond about 10^30g of mass or 20^26 watts is a hard-ish limit minus FTL travel.
Other limits? I'm not aware of any other hard limits.
Once I hear people proclaiming the end of available mass or energy...and using sane values like 10^25 g or 10^17 watts...then we have something to talk about in terms of scarcity. Until then...it's basically all artificial limits and technical problems (some of which aren't solved yet).