As someone who mostly uses video in the context of online systems, MP4 (H.264) is the closest we have to a universally supported format on the user's end and it has been for years. However, it's complicated, patent encumbered and very inefficient by modern standards. And even MP4 isn't 100% universally supported.

Once encoding and decoding at reasonable speeds is possible -- we're not there yet, but there has been a lot of progress already -- I see no reason that AV1 shouldn't render H.264 and similar legacy formats obsolete quite quickly. The new format doesn't need to be perfect at first, just good enough to use with mid-range to high-end devices. Then the openness and improved compression/quality will make it an attractive alternative, with legacy formats reduced to being just a fallback for less capable devices. One generation of devices later, almost everyone can play it. Two generations later, you probably have hardware support, but at that point it's more about efficiency things like reducing battery drain on the viewer's side and hardware costs on the encoding side, because the format itself has already won.