You can not hold your fiats for next 20 years but at the same time you can save and hold your bitcoins for next 20 or even 50 years also. That is the beauty of bitcoin and it will get multiplied minimum thousand times in next 20 years.
Thats not certain bitcoin itself will be unchanged but this is why people compare it to gold which is inert. The main thing is if bitcoin last 20 years it will have proven itself and so be more reliable to a wider section of society. This implies far greater worth then now as bitcoin coin growth is greatly behind natural human population growth so the price rises
Sell them for a ton of cash "literally a ton
"
If bitcoin is around in 20 years it will be superior to cash at that point. So you are saying you'd swap all of this superior standard for some simple cash which is easily degraded compared to a monetary standard thats last 20 years.
Dollars dont really last 20 years as they keep printing far more then the population of the country rises or the productive capacity of the country rises, this means the standard of dollar is constantly falling. Over 20 years that fall is quite large, possibly we see 50% loss in value. It might not be true but if bitcoin was almost perfect in comparison, it'd be seen as much more reliable. The other scenario is bitcoin destroys itself or overtaken by hack attacks or technology itself allows it to be corrupted. I dont know how but it'd quite astonishing if bitcoin really did last 20 years.
The scenario I expect is technology replaces and surpasses the bitcoin concept, perhaps in a postive good way I dont know