You're right only a persons discretional income should be invested in Bitcoin and not the other way round , investing an entire life savings in Bitcoin all at once is financial recklessness which could also lead to financial discomforts. When a person invest his entire savings in Bitcoin,at that point he's now financially vulnerable because he's empty without any funds to sort out his Bills and other of his necessary needs. Since he is a long term investor,there is no points pouring all of his funds at once in Bitcoin of which is very wrong from the starts. A smart thinking investor would only invest in Bitcoin appropriately without overdoing it or stepping out of his financial limits.
I agree with you, one of the most important things in Bitcoin investment is to invest with financial balance, if you invest aggressively, an investor will panic very easily, because when the market is down, that person will not only suffer from mental stress, but he may also make any wrong decision due to excessive panic. But if he continuously invests in Bitcoin through DCA, he will never panic during volatility, because he is increasing his Bitcoin holding at an average price through continuous purchases, so volatility will never scare him too much. So, before investing, you need to know about these volatility issues, and instead of deciding to invest aggressively, deposit Bitcoin in DCA through prudent income, and keep emergency funds ready to protect the investment in the long term.