It doesn't matter what things cost in the past, just what they will cost in the future. Past performance does not gurantee future results.
Then why all the talk about Bitcoin's past performance? No one is arguing that investing in Bitcoin turned out well for people in the past. As you correctly pointed out, that doesn't begin to begin to imply that investing in Bitcoin is smart *now*.
If you make an estimate of what the future price of bitcoin will be, then it might still make sense to buy bitcoins. Saying "They went up 10x every year in the past" is not a valid argument. Saying "They will be about the current price later" makes them a valid savings instrument, saying "They will grow another 10x because of such and such reasoning" makes them a valid investment instrument.