And they were right, well, sort of: early 2016 had some awesome buys (notably ETH). Fact is, if you're looking at investing during a bubble (or half-bubble; we don't know how much further this run will go), you're already too late. Nov 2018-Mar 2019 had some amazing crypto deals. That was the last time I dipped in. Though I apparently did not buy quite the right thing, I'm still well in the black.
You should also consider the typical long-term strategy, dollar-cost-averaging. You buy when you can and however much you can.
There is truth to the notion that "stonks only go up", at least the good ones, and the same applies for other asset classes like resources (gold, bitcoin, ...).
So you buy now according to your investment/retirement plans, sure it may seem high, and it may fall back down to a lower level for a while. So you buy some more on the way down if you can. Eventually it'll re-correct and continue the upward trajectory due to supply constraints. And as it goes up, you still buy more. Because you keep adding to your positions and reinvesting earnings.
There's just no guarantees that BTC goes back down below 20K ever again, for instance, or ETH below $1000. Or maybe they do and keep crashing and completely fall out of use and the entire cryptocurrency movement goes bust.