When there are a number of Wikitreers (I am one) who personally knew people who were born in the late 1800s, and could legitimately say they had personal knowledge of the person, maybe 100 years is a bit soon.
My grandparents were born in the 1890s, and I knew them -- Grandma more than Granddad, but I still knew them. My parents-in-law from the first time around were born more than 100 years ago, as well. I sure knew them. Any Wikitreer who was born in the 1930s and1940s will likely have personally known people born more than 100 years ago.
I dislike that as an only "source", so I understand the desire to see it go, but how can it when it can be legitimate. Maybe figure out the birth decade of the oldest registered member, and go x years back from them -- making a presumption that they knew their grandparents? But, then, what about those people who knew their great-grandparents? My cousins kids' kids did - and those kids were born in the last 30 years.