Thus they argue that you should not trust your things to other people but do it yourself, because, barring major illness, you can reasonably assume that you will try to do what is best for yourself in the future.
Nope. At best you merely know what your current self intends to do in the future. Whether you'll do them then, or whether you'll even still want to do them then, or be able to do them then, or have regretted ever wanting to do them, and many other such things, we don't know.
>because, barring major illness, you can reasonably assume that you will try to do what is best for yourself in the future.
Furthermore the phrase "some things" does not actually mean all things.
So again, trying to put it another way to clarify anything that may have been unclear:
You can be reasonably certain at the time that you need to make a decision that you will try to do what you believe is the best for you. I will now not go on to make an exhaustive list showing every way that you in the future may not do what is right for you because it is normally understood that there are numerous ways in which the reasonable expectations of people can be countered but nonetheless, the reasonable expectations will still tend to hold more than any of the countering scenarios. This is why they are the reasonable expectations, while the countering scenarios are the notable exceptions that often show up in sensational news reports and fiction.
And finally I should note this comes down to the principle, which I would have thought was clear from my previous post but evidently not, that if you can reasonably expect that future you will try to do what future you thinks is best for them, you can just as reasonably expect that the future corporation you are buying a service from will try to what the future corporation thinks is best for them, without giving much consideration for what is best for you.
I do not know what you think is best, but I would normally trust future me over future corporation not owned by me but providing me a service, I guess I'm just funny that way.
You absolutely can know something about what someone will do in the future. You gain those insights by looking at their past and present behavior and the behavior of others in similar circumstances. Hackers tend to be pretty good at pattern recognition and can often see where things are heading. Enshittification is a pretty smart bet.