If a person has a reasonable source of income and is willing to invest, then he should never delay investing. It is best to start investing first. You can do everything after investing. If you want to learn everything beforehand, then it will be too late to invest, it will never be the right decision. You have to start investing first and then you can learn a lot.
The point is not only to have sources of income, but the point is discretionary income. Someone can be earning a good salary or wages, but that doesn't always guarantee discretionary income whenever they receive their salary or wages. In fact, there are people whose salaries are not that big, but with the little they receive, they always have discretionary income when they get paid.
As you have said, waiting to learn everything about Bitcoin before someone starts investing in it is just a waste of time because you will never have the opportunity to learn everything about Bitcoin in a short time. Bitcoin is volatile by nature, and the price can just continue increasing. Waiting to learn deeply about Bitcoin may cause someone to miss opportunities. As long as someone has basic knowledge, it should should start Bitcoin investment and continue learning among as it continue accumulating.