The Not-Fazbear company got a lawsuit from our favourite company which then causes them to get some financial issues

They then have to scrap Cosmo to sell parts for money and its much easier to take care of one then two robots amiright?

Ribbon who is in town that week visits them just to see Star crying. Ribbon then becomes the therapist for Star as they both tru to cope with the loss of Cosmo

Now that might be a good place to end it

But might angsty mind said no

A terrible accident happened in the circus that was caused by Ribbon unintentionally,but thats already enough to cause him to get reprogrammed and reset for new protocals.Leaving star alone as Ribbon no longer remembers him