While I can agree there may be some aspects like leagues eventually folding before you've gotten the full value of the pick, I don't think that's generally why people do it.
I think people in fantasy accept it fairly blindly because the NFL does it that way. However, I think the main reason the NFL tends to do it that way is that general managers who are worried about losing their jobs are willing to pay a premium price to get a future asset now, when it may help keep them employed.
Because of GMs who are willing to do that, the trade market ends up set there. Then everyone as to pay that price if they want to do such a trade.
Then in fantasy, enough people just follow the NFL's lead that it becomes the market price as well. I don't think it should be though. Barring actually expecting a league to fold soon, I think this year's and next year's picks should be regarded as the same value most of the time. Potential quality of the draft classes, and of course where in the round the future pick will end up should be the driving factors.
I agree that NFL GMs have different incentives, and therefore devaluing future picks might be rational for them but not for fantasy owners.
I disagree that most fantasy owners devalue future picks just because they're aping NFL execs. I think it probably has a lot more to do with psychology; more specifically,
construal level theory (explains how priorities change based on temporal distance, and goes a long way towards explaining why draft picks change in value in predictable ways throughout the season) and
hyperbolic discounting (bias towards immediate payoffs over delayed payoffs).
Some sort of devaluation is rational. If your league involves money, then you have to consider the time-value of that money. You also have to account for the chance of the league folding, as has been mentioned. Finally, if you played 20 seasons and won a championship in year 1, you would have 19 more years to enjoy the status that resulted from being a champion. If you won in year 20, you would not be able to enjoy being a champion at all. Winning earlier is preferable to winning later because it gives you more enjoyment over the lifetime of your fantasy career. Accounting for all of this, it makes sense that, all else being equal, a pick this year is worth more than a perfectly equivalent pick next year, and even more when compared to a pick the year after. The question isn't whether there should be a discount, the question is whether the discount should be as large as it is in practice.
And in practice, I would say that my experience suggests that the actual pick discount is much higher than it should be.