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.
I do lay emphasis in this thread and also in the forum about not letting expectations take over us, same time we understand that we are investing, of which investing is simply accepting uncertainties that might occur on such asset (Bitcoin), that the need you (JJG) do emphasize on the word Disposable income to invest. We are not getting too optimistic about the price we just speculate, deciding based on the value, past history which also doesn't guaranteed the future of new price movement, of which the sole reason I advocate for Plan in all my post which any investor would act within to safeguard himself.