There are sooooo many ifs that you have factored, perhaps wrongly, into your surety. 

But my response was to btc being useful for a retirement account.  I called it a currency.  Is it a currency or an investment? 

If it's a currency, then the best thing for it would be price stability.  In that case, btc would be absolutely horrible to sock away for retirement, because inflation would eat it away--and I mean inflation of fiat, because I do not expect the major currencies to collapse in our lifetimes, and since things are priced in fiat and btc is largely purchased with fiat, btc is tied to fiat.  I could be wrong. 

If it's an investment, what justifies it's price?  It represents no earnings as a stock does, or debt like a bond does.  It's major advantages are as a currency: it's quick to transmit and the fees are very low.  It's not even a physical thing like precious metals, art, or stamps. 

You might be sure btc will go viral, but I'm not so sure it'll be around in 20 years.  Again, I could be totally wrong.