Of course MIDIs have more creative possibilities, but most people are simply listeners, and the only way to allow them to hear what you intended when they have completely different hardware and software is to provide your music in a format that sounds the same everywhere, and that's what MP3s and other recorded formats provide.
Making MP3s isn't difficult either. Just take the final MIDI and record it with all of your best soundfonts into a raw wav audio file (redirecting audio input/output via software isn't difficult, so you can get a very pure sound), then compress it. Now you can share the same exact sound with all of your fans on the Internet who don't have the ability to use MIDI soundfonts.
See the idea now? MP3 and what not are simply better for distributing your music to listeners so they can listen to it as authentically as possible, because most people aren't even savvy enough to install a soundfont, even if they have the hardware. And since most people don't have that hardware these days, it makes MIDI even worse for distribution/publishing.
If you're really into music synthesis though, you should also look into the tracker scene. Instead of using soundfonts, trackers let you craft music from any sound sample (you can use pretty much any sound as the basis for creating an "instrument") and doesn't require any hardware support from the soundcard (although most tracker programs let you plug in MIDI keyboards as input devices). The software then stores your tracks and the sound samples into a "module" file, which is like a compromise between a MIDI and a recorded audio file.
Like MIDIs, module files are relatively small and don't contain redundant information, and you can edit other people's music, but like MP3s and recorded audio, they'll sound pretty much the same no matter where you play them. However, they require people to install special players (or at least, something like WinAMP), and most people would rather just download a larger audio file these days and not mess with any programs or anything, so it's kind of like the soundfont problem.
Another way to look at module files is that they're more similar to the way music was used in older console games, back when they had tiny cartridge formats and had to save space. In those days, music was often built in the SPU from audio samples stored in the game's memory using SPU assembly code. That way, music in different games weren't limited by the console and could have any sound they wanted. Likewise, module files build music from audio samples stored in the file using the patterns constructed with the tracker software and can achieve all sorts of sounds.
I guess, the easy way to think of a module file is like a MIDI with the soundfont built in.
You can read more about trackers here:
http://en.wikipedia.org/wiki/TrackerBut yeah, there's a lot of interesting stuff out there if you want to be creative. It'll always be there too, so don't worry about soundfonts and MIDIs disappearing, because they won't. Just don't expect non-creative people to be using those things, or for them to be as commonplace as they used to be, because they're becoming more and more just tools for creative people to use.
And yes, I used to listen to MIDIs back in the day when dial-up was the norm and a 133Mhz Pentium was considered a pretty decent machine (I even had a MIDI keyboard), but nowadays I'd rather listen to an MP3 or a module file and know that what I hear is pretty close to what the artist intended (albeit, limited by my laptop speakers and integrated sound chip).