A solution that is 100% interoperable with the existing widely deployed solution but also offers substantial advantages can have good chances to coexist with the old solution for years if not decades, supplanting it only slowly.

Examples of success: monochrome TV -> color TV, landline phones -> mobile phones, the Windows 3.x -> Windows 9x -> Windows NT/2k/XP transition.

Email has a really well-working transport layer. The UX layer can see some innovation while staying compatible.