It solely the decision of any investor to decide how much he is willing to invest in Bitcoin or any other asset. Bitcoin have high return proportional to the investors Level of Accumulation of it (stash) which is guaranteed in a long run, it simply a matter of discipline, if the investor is disciplined there is nothing that can shift his focus.
Bitcoin's price performance is not guaranteed in the long run to go up - even though many folks (including myself) invest into bitcoin based on presumptions that it is more likely than not to go up in the long run... so each of us needs to establish our position size in accordance with a variety of factors, including the factor that BTC's price is not guaranteed to go up.