Do you think 1 Bitcoin can be good for retirement...let's say 20 years from now?
After 20 years maybe bitcoin is already $50,000 from that time or even bigger. I guess it's already enough to save one bitcoin for your future keep and retirement. But if you can add some more just keep on adding that will be the best thing to do. Because you will do nothing but will just let it keep at your cold storage.
Having more than what we need is definitely better than having less, if the only thing you can afford for your retirement at the moment is 1 BTC that is fine you are ahead of most people in the world but if you can afford 2, 3 or 10 then by all means do it, this is especially true if you are young since the age of retirement is far away and your investment is bound to multiply many times over.