It's inevitable only if both sides are around long enough to reach it. Allowing one (namely, machines) to singularity first could result in Synthesis... or it could result in our total annihilation/obsolescence at their hands. We need them, not the reverse, and all it would take is another Dalek Heretic mindset to spring up in their ranks (due to a math error no less.)

Perhaps your Shepard is willing to roll the cosmic dice on that, and that's fine. But hopefully you can understand why other Shepards may not want to.