What major problems are crypto and btc facing in the future? One I can think of is taxes. Will there be offshore tax havens set up for people to exchange their btc, eth and other crypto into fiat without paying taxes? It's not really decentralized if you have to pay the government a cut of it. What if you are given btc, is that still taxed?
How to spend your crypto? Will you wait until retailers accept your crypto of choice? There are options like tenx card, but what are the tax implications of using that? Even if retailers accept crypto, say monero to buy groceries or pay your rent, will you not still have to pay taxes on that?
Another problem could be power outages, internet disruption. What if there is a power outage for a year or two? What if the internet is disrupted globally?
The idea is to never go out of Bitcoin. Obtain bitcoins by offering services and products, and buy services and products in exachange of bitcoins. If you have noticed, the circle is closed now and there is no need for fiat.
If you need to go fiat, don't expect to not get into trouble. If you want fiat pay your taxes and sleep well, there's no way to bypass this. Look at Paradise Paper et al, it will always leak so it's stupid, just pay the damn taxes and deal with it, or don't ever sell.