That's a weak point. If you say you want to think long-term, which of these do you mean?
1. I want to make plans that have long-term effects I consider desirable.
2. I want to make plans that are similar to those of other organizations which describe themselves as thinking long-term.
On the assumption that we're talking about #1, a record of actual longevity always trumps the empty claim of future longevity. You can say whatever you want. Saying you're going to have a long-term impact doesn't mean you're going to have a long-term impact.
But surely there must be more to the story than survivorship bias? Can intentions towards long term thinking have the desired effect? If so, it's reasonable to consider what strategies and tactics are most helpful.
I think you're being a bit presumptuous about institutions whose lifespan have yet to be determined. It's not like practices from successful long-lived institutions were ignored.
The examples you cite suffer from a reality that in many ways those institutions survived for so long in name only. They have transformed significantly enough from one generation to the next as to be largely unrecognizable as the same entity beyond comparative trivialities of language/ceremonies etc. If you pulled together snapshots of them from various points in time and put them together, they'd often be at cross purposes.
Of course, the same criticism could be leveled at the post-WWII institutions, but those institutions were defined more by their purpose than their mere existence, so it was "pioneering" in terms of the effort to maintain said purpose, even if in many ways they've failed. I do think looking at the ways they've failed despite design intent and historical knowledge applied to them is potentially very interesting.
1. I want to make plans that have long-term effects I consider desirable.
2. I want to make plans that are similar to those of other organizations which describe themselves as thinking long-term.
On the assumption that we're talking about #1, a record of actual longevity always trumps the empty claim of future longevity. You can say whatever you want. Saying you're going to have a long-term impact doesn't mean you're going to have a long-term impact.