Well suppose Bitcoin is expensive but in that case no one is forcing you to invest in Bitcoin rather invest as much as you can in Bitcoin as per your ability. One should not invest one bitcoin but invest and hold 0.002 bitcoin amount. No matter how expensive Bitcoin is, if you want to invest, you will invest no matter how much the price rises.
I think each has their own trading strategy in that case. However, when the market starts to bull and the price starts to rise slowly, for example in a week, I think buyers don't count anymore, meaning that the price that is running/rotating is bought so that he can sell it and get a little profit, he feels it is enough and he plans to rotate to another while waiting for another price correction. My asumtion, in this moment the current price will be said to be still very cheap if at some point BTC will positively rise to a price of 100K in the future.