Just as a sensible income is needed to start investing, 2 more things are needed
1 Basic knowledge about Bitcoin
2 Faith in Bitcoin
I wonder what type of sensible income you are talking about because to the best of my knowledge, their are so many guys out there that wish to invest in Bitcoin that are earning a sensible income as you called it but still can't figure out their discretionary income, what an investor need in other to invest in Bitcoin consistently is a discretionary income and the will to hold for a very long period of time, because if he has the will to hold for the future, he will put down measures that will make him develop a strong holding hands, like having an emergency and reserve funds in place to protect his investment against any unforseen circumstances that may arise in the future.
When it comes to investing bitcoin, no doubt, having discretionary fund is very important, but I don't think this is the only thing that makes people to be able to invest bitcoin because their are some people who have this fund but they still find it difficult to invest bitcoin.
Having a good understanding of bitcoin is also important and also good panning sense. Understanding is the main thing, with this one will be able to manage the discretionary fund to invest very well. The reason why investors are very consistent in their investment is not just because the fund is avaliable, it is because of the understanding they have about bitcoin investment which is being considered as a major target to meet with .