Bitcoin is a good investment, and I believe it would be worth more in the future for anyone with plans using it as a part of their retirement plan. Though I would not recommend going all in in Bitcoin, try invest in other things as well, this is life we are talking about and nothing is fully guaranteed. So invest wisely.
I agree with you, this is a piece of wise advice, many people who consider Bitcoin as an all-in asset end up regretting it later because no matter how good a thing is, there is no telling the future. Maybe everyone has good hopes and believes that Bitcoin will go higher in the future and they will benefit, but this is all just hope, I'm not sure why, they put all their investments in Bitcoin. It's not wise to go all-in, one should look at other things besides Bitcoin and wise people are the ones who make good decisions for the future and don't depend on just one thing but always have other options.
I have seen people make mistakes themselves and later blame other people who did it on the advice of so and so and if they succeed in something they take all the credit and say yes, it was my idea, it was a wise decision, I did it and I succeeded. If you hold yourself responsible for success, you must also hold yourself responsible for failure.