We do live and exist in a fiat currency debt bubble and part of existing in a fiat currency debt bubble is the reality that booms and busts are perfunctory per design of the system.  

That being said, the majority of my assets are in real estate, but I also contribute to a Roth IRA, S&P Fund, hold shares in Fortune 500 stocks, and recently have invested in crypto currency.

US currency downfall as the fiat currency.  

There's also the possibility, and I know many people think it's unimaginable right now, is that the developed world total debt will become so huge that the powers that be decide to wipe everyone's slate clear, and declare a single, new currency.

Currency may become irrelevant.

I'm 68 and "retired", so do not want to commit to a BUSINESS and am not sure RE can be done as a "hobby" with great success.At present I trade currencies and options, but that is capital intensive and inconsistent income-wise.

Also seems smart to have an income source from another country as a hedge to currency fluctuations.Anyone out there feel different?

Now concerning market risk of a deployment, natural disaster, collapse of the currency or running out of oil or anything.  

If cryptocurrency is just another currency, then accepting it is no different than, say, accepting Euros, Canadian Dollars or British Pounds for your vacation rental then converting to USD.

I know it can be done because I've sat on google translate to annotate German tax returns into English, along with past-dated USD currency conversions to get a loan through, but yeah this is one where you need to shop primarily the competence of the lender, and shop it hard.