From an economic standpoint, I would say cash to me becomes trash when its exchange rate gets way higher on the market and incurs higher interest rates.
At that point, Bitcoin or alternative means of exchange like gold, can become more used while cash becomes for meager necessities that don't cost much.
That is why cash cannot be compared with other assets other than cash, because we will never find any advantages or similarities in cash when we compare it with other assets such as gold, Bitcoin and others. So cash will only be used for ordinary payments which are still often seen in traditional life so it is natural that some people consider it as waste.
Cash will also tend to become trash if cryptocurrency gets accepted and regulated and gets used more often, having become a part of society and trades. It may still exist but it might become of less benefits to those in more urban communities that has developed and operate mostly on cashless economy.
And if the use of cash can be reduced in society with more other options being used by more people, I think cash could really become waste because very few will use it for anything. But because currently many people still use cash in life even though there are other options such as gold and Bitcoin, I will not immediately consider cash as waste because I can still use it to buy things that I need in my current life environment. However, I also wouldn't be surprised or deny this if it could actually happen to cash in the future.