Totally agree, @charlie. If there is a desire or need to cross-reference legacy observations with more recent offerings, that could always be done under the contemporary account of the person managing that legacy account.
Say, for instance, someone rediscovers and uploads a new observation of an Eskimo Curlew. A useful note that I might add (as @gcwarbler) might be something to the effect of “The only other record of this species in South Dakota appears to be my grandfather’s observation of this species in the same location back in 1954. See [this observation] from [MyGrandfather’s legacy account].”