So many factors; it's really a "try it and see" thing. A big part is "what will the rider be doing?" The use case of "riding the boardwalk at the beach" is a lot different than "descending at 40mph" and a lot different than "snowy trails on a fatbike." While a modification may work fine for some situations, it might not be great for others. I imagine a Schwinn Sting-ray with frankenbike "double fork" extensions (think 15" of trail). That would be cool for riding around the cul-de-sac in the suburbs. Down a 7% two-mile hill at speed? Ummm, maybe not for me. This question of stem length seems very conditional on the use case. It's a pretty wide spectrum between "barely functional" and "truly optimal." User requirements do vary.