Nothing, the end result has always been Magical Girls becoming witches regardless if they fight or not.
When a Magical Girl is made, their Soul is removed from their body and made/placed into the Soul Gem. As they use their magic, the Soul Gem clouds in which they need Grief Seed to purify. If it's not purified, it will become a Grief Seed and the Magical Girl will become a witch, shown when Sayaka transformed in Episode 8
After Sayaka became a Witch, in Episode 9 Kyoko is seen using her magic to preserve Sayaka's body. We can speculate that without a soul, the body starts to rot and the Soul Gem is actually preserving the body on its own.
This is somewhat hinted in Episode 10 in the timeline Homura and Madoka defeated Walpurgisnacht and Homura was asked by Madoka to kill her (the final timeline of Moemura). In this timeline, we see that both of their Soul Gems haven't been fully tainted as they didn't have the same pattern as Sayaka's
Since all Madoka and Homura was doing was laying in the ruins not using any magic themselves, we can assume that the Soul Gems continue to taint as they maintain a Magical Girl's body until it starts creating those strange patterns in the blackness that Sayaka's Soul Gem did just before she transformed.
Alternatively, since a Magical Girl's body just stops responding when they are separated from their Soul Gem (as demonstrated when Madoka threw Sayaka's off the bridge and it landed in a truck that drove past), one could also assume that the ability to control their body also slowly taints the body, including talking which is what Madoka was doing before Homura kills her.
As such, even if you tried reneg on your side of the contract, I speculate that the end result would still be the Magical Girl becoming a Witch
What about after the series?
Post Series:
In Madoka's Universe, still had Magical Girl's purifying their Soul Gems though those black cubes Wraiths drop (shown when Homura is talking to an Incubator about the witch system)
Post Rebellion:
In Homura's Universe, we aren't 100% as to what has changed with Magical Girls, but given that Homura has given the Magical Girls normal lives and stripped them of their memories (and power as evident with Sayaka's confrontation) and that she had the power to erase the Incubator's from existence but chose not to instead re-purpose them to do Madoka Universe Magical Girl Work (dealing with Earth's Curses), I would assume that she has made it so that so that Magical Girl's won't become witches via inactivity, otherwise this might cause the Law of Cycles of act and reclaim Madoka to deal with them.
