I think MP3 is likely to be on pretty much all players for years to come. Some old formats become standards and this is one of those. Other examples are GIF and FAT (or FAT32) and AVI. These are all outdated formats and nearly everything that can support them does.
Last year I found a site that had a lot of very old TV shows that I hadn't seen elsewhere in .rm format. I hesitated to download them even though I wanted the shows but I did and sure enough I have players all over the place that'll play them just fine. Probably the only thing more out of date than .rm are moons cut into the doors of outhouses.
Barry