We can hardly predict the price of what Bitcoin will be in 2 weeks. I doubt we can predict what it will be in 20 years. My advice, save some Bitcoin and hope for the best, also have other retirement options as well. Don't put all your eggs into one basket as the old saying goes.
Do not put all your eggs into one basket. This is a good quote especially for long term financial planning.
In fact, we just know about bitcoin from 2009 until 2015. We can't say for 20 years up next.
Anyway, it's not safe to just hold BTC for your retirement, and of course you already know it.
BTC is good for investment. But you cannot just hold BTC, if you buy bitcoin on Jan 2013 (@ $1,200) and waited until today price @ $327
you will regret to put all your investment just into BTC.
But on vice versa,
someone who put everything on bitcoin in 2009. He already become millionaire now. And this people will keep holding BTC forever.