Bitcoin as a retirement plan for my parents

[Ziskinberg]

Convincing them to invest in bitcoin for their retirement plan will not work if not supported with real testimonies. At least, you should be profitable with your investment first so that you can easily attract them and make them believe that bitcoin is never a scam but certainly a profitable source of income. That way, once they invest, at least  it’s their own decision since the money are theirs and not because of your own will.

Well, we can say that the most convincing power is to show other people the output of your crypto investment first which I agree with that. Maybe, if OP has it, possible his parents will be convinced. But for me, I don't make decisions about what my parents wanted with their money. Let us say that Bitcoin investment is profitable but can't say that all people who invest in this area went well, some ended up losing their money because of being impatient and lack of knowledge about the market. And this will possibly happen to his parents.

[coupable]

In general, it is never advisable to invest in what we fear of losing, and in this case, the money is not on the OP's property, but on the property of his parents. The best that can be done is to convince them that bitcoin is one of the successful investments in the long term, while pointing out that it is not possible to set a specific time.
Also, we cannot blame anyone for his fear of what might happen in the future, especially if he lived through market reversals and was aware of it. On this basis, it can be proposed to invest part of the savings in Bitcoin (at least as an experiment) and then leave them with the option to continue investing more money, or to be content with that amount, or to abandon the whole idea.