I admit that I haven't been to Brussels in ten years, but Midi is a modern station with many business travelers. A run-down European neighborhood (at NOON) around a rail station is NOT like a rundown neighborhood in an American city, and what little street crime there is in Belgium is not like street crime in the US. I would walk.
However, if you are staying in Brussels for a few days, and using the train daily to visit other cities, you need to evaluate whether you should keep this hotel reservation, just as a matter of personal comfort.
Notes: I grew up in Manhattan, NYC. In 1989, my fanny pack was pickpocketed near Termini Station in Rome. I switched to Rick's inside belt pouch and have never experienced any crime in 50 subsequent trips to Europe.