[bzzzt]

MIDI was designed as a way to link up a controller like a piano keyboard to a synthesizer and transmit the notes being played. In that area it's certainly not obsolete: synthesizers still transmit and receive MIDI messages and sequencer software still records them. It's the only sane way to edit and view note data because you can't edit audio easily to change a few notes.

As a way of transmitting musical scores it's obsolete, but mostly because it was really hard to get MIDI soundtracks to sound as intended because there were so many different synthesizer implementations.