Please remember the immortal Pentax K bayonet mount. To the best of my knowledge, and the use of my Pentax K mount lens collection, all K mount lenses can be used on all current K mount cameras with, of course, some limitations. With the proper adapter, the excellent 42mm Pentax lenses can be used on K mount Pentax cameras.
As far as Canon's abandoning its earlier mounts in order to accomodate faster lenses, all of my K mount cameras are happy with my Pentax 50mm f1.2 K bayonet lens.