It wasn't that 40 years of policy won't be enough to convince people to trust in said policy - it's the "promise" of a 40 year policy that isn't enough to convince people to trust in said policy.
Just telling people to trust the policy will be held strong for a least the next 40 years will not automatically convince people to quit their jobs and all the other possible outcomes of UBI. People who've grown up and spent a major portion of their life without UBI will likely remain skeptical, and mostly will not depend wholly on it for income.
Rather, it will, possibly take a generation of people growing up with UBI all their life before we start to see the actual societal changes UBI might affect. By then, it might be too late to make a change since there will be a generation of people who've come to depend on the government's hand for their basic needs.
1) Even now, plenty of people take into account the possible dissolution of Social Security or (more likely) changes to its benefit formula in their financial planning.
2) Longer time horizons could always have a hypothetical effect. You say a half century of a policy can't predict a hundred years of a policy; someone can just as well say a hundred years of a policy can't predict two hundred years of a policy.
In the long run, though, we're all dead, and the economic landscape is going to be so radically different in a century that it's pointless to predict how a particular policy will fare then. Someone in 1934 making predictions of the effects of Social Security in 2034 would be so far off as to not even be wrong. See, for example, Alf Landon's criticisms of the program then, which don't correspond at all with what its critics today don't like about it.