Bringing folks 20-30 years into the future where writers and the audience could be more concerned with what the future looks like in terms of tech and other nuances may distract or pull away from time spent on storylines.
While cool, flash-forwarding that far into the future could prove problematic for This Is Us.
The major difference between what the show did in Season 1 vs. Milo Ventimiglia’s pitch is that both eras have existed in the first case.
Of course, it’s always possible that the show could write around stuff like that and keep flash-forwards brief, and even if they did work in some future stuff, it may not be all that bad!