In fact, investment is not as easy as we think, just as before investing, the investor has to plan separately about the investment, so the investor also has to think about whether he will be able to invest consistently.
It's not compulsory for a new investor to start his bitcoin journey with consistency on carrying out his DCA because it's not everyone that has a stable discretionary income. Provided the new investor has a discretionary income that can enable him buy bitcoin and hodli without being consistent, it'd fine. The most importnat thing is that he is no longer a no coiner.
Some people income is very small that they hardly get a steady discretionary income from it, but they can invest into bitcoin whenever they can with the little discretionary income they can afford, either they got it by saving little by litle or from a bonus at work of a gift from a friend. They can buy bitcoin and when they start learning and understanding the potential of bitcoin that it increases in value exponentially. They can be motivated to improve their income by looking for a second means of income and when that is achieved, it will boost their discretionary income and they can be consistent with their DCA for 4-10 years and above.
About investing what you can risk, the context of your description makes it appear like you are referring to bitcoin gamblers.
Investing with the amount that you can afford to lose simply means that you should only invest with part of your discretionary income and don't put all your hope in bitcoin to be a life changer for you in the future and invest with all your life savings so that, if bitcoin price goes against you in the future, you will not regret for the rest of your life in poverty. Also it will make you set up other stream of income so that you don't put all your dependence on your bitcoin investment as the only thing to survive with in future. The future is fill with uncertainties.