Basti and Jouvet (1988) describe a suicide attempt using 4500 mg of modafinil; the suicidee survived with no long-term effects but temporary nervousness, nausea, and insomnia.
That's 45 doses of modafinil and she was fine afterwards. 45 times the typical dose of caffeine would be lethal. Sure, you can argue from ignorance about potential long-term effects, but that argument works for anything that wasn't invented before you were born. Based on the evidence we have now, modafinil appears quite safe.
Edit: You are the first reply that didn't start with "because." You broke the pattern! >:(