Bitcoin currently sits at about $39,666 (for one Bitcoin) at 12:30 pm EST on January 7, 2020. The all-time highs are happening every day (even every hour) recently. None of the crypto prognosticators predicted so much moon so fast. I do believe that the present bull-run for Bitcoin is supported by fundamentals which are
- institutional investment,
- economic uncertainty (extreme U.S. government debt, extreme dollar printing) so that Bitcoin is viewed as a safe harbor,
- limited supply.
However, most other cryptocurrencies which have seen a rise on Bitcoin's coat-tails as well do not have the same fundamentals supporting them and if one wants to apply the labels of "irrational exuberance" or "bubble" to them, the labels would be more appropriate than to Bitcoin. Actually some of the larger market cap altcoins like Ethereum, Bitcoin Cash and one or two others have some institutional investment in them as well, so their gains may be sustainable and credible, but if you're seeing lower market cap coins making huge gains, sell on the highs because those altcoins are less likely to hold any recent gains because there aren't any supporting fundamentals, in most cases, to justify large price gains. (Of course you should keep/hold any coin which you believe in, I'm just saying for investment purposes most altcoins will not keep up with the Bitcoin price increases.)
Bitcoin always has price pullbacks and that should be expected, but the trend is always going to be up until we get closer to the last minted/mined Bitcoin in the 2100s.
Check out: The Essential Guide to Bitcoin Mining: A Cryptocurrency Tutorial