I believe that will never happen.
I think that, since the antichrist rome tries to destroy any institution defending the Catholic Tradition, it will not allow an agreement with Bp. Fellay. Obviously, the current situation is damaging the SSPX more than a sell out.
This may not sound logical, but look at how much damage the current situation is causing: there is not (yet) an agreement and the Society is divided. Those who oppose Bp. Fellay's betrayal are divided as well. Inside the neo-SSPX are divisions too, but they are not easily visible (yet). It is known that some priests do not agree with Bp. Fellay's politics, but they have not decided to openly act against the superior general.
If an agreement were reached now, I think many priests would leave the Society and many lay faithful would follow them. But if the agreement is delayed a few years, then the number would decrease. That's what I think.