That's one of the theories (not counting the auto-inclusion of KYUBEI IS MOTHER****ING EVIL theory).
This is also reinforced by... (spoilers and potential NOOOOOO)
But think of this though. It's probably not the only way witches are formed. Mami mentioned that if you leave familiars out for too long, they turn into witches (are familiars different from minions though?).
There's a fair amount of terms that still need more explanation. I'm sure most of it will be answered when Madoka/Sayaka turn into mahou shoujo or when Homerun-chan actually shows her transformation.
I'm iffy on the grief seeds = mahou shoujo trinket. They all look a lot different from each other, not to even compare to the grief seeds. Unless they morph. But then that implies there's been... quite a lot of mahou shoujo exhausted/dead.
EDIT: I forgot to mention!
The ED card of episode 2 also supports the theory that Firzin mentioned. Something about that guy fighting for fuel, which can be compared to the mahou shoujo fighting witches for grief seeds (refuel etc.).
EVERYTHING GOES FOR EVIDENCE.