[wlesieutre]

It's not "because it might be used too much," it's "because developers might bury tons of functions where no one will ever find them, because that's easier than designing a learnable/discoverable UI."

Whether you agree with the argument depends on how much faith you have in developers/designers to understand how average users will experience and learn their software, and in whether they might go for the lazy "shove it in a contextual menu" solution even knowing that it's questionable at best. 3D touch is supposed to be a "power user" feature; if you don't know it's there you can get by just fine. It'll take you an extra half second to open an email.

By holding it hostage on some new products, Apple can hope for more developers to say "I'll use this for time-saving shortcuts and other convenience features since the iPhone SE users don't have it." Once best-practices for that are more ingrained and many apps have implemented it in a way that lines up with Apple's vision, there's less risk of buried features when they eventually include 3D touch across the lineup.