Cost of "mining" a bitcoin still costs more than market value, that fact alone should be all the technical support you would need. The only way bitcoin fails at this point is if Blockchain technology itself were to fail, and I don't see that happening. Scarcity of BTC is increasing exponentially, the price will begin too.
If you aren't already a believer, I don't know how to convince someone at this point in the game. In a few weeks, our grandmas are going to be asking about Facebook's LIBRA coin. Libra has massive potential to pave the way for crypto-favored legislation. If BTC does dip with the release of LIBRA, just buy more, because it will only be temporary.
If you can find someone with a better history of calling BTC right, let me know.