The Synthesis ending is in and of itself a trick. Why would the Catalyst, an AI that seems utterly convinced of the inherent superiority and inevitable dominance of synthetic life, 'water down' synthetic life by splicing it with organic?

The truth is, the hybridization is never intended to be permanent. Instead, the new DNA created by the Crucible naturally favors evolution towards a fully synthetic state, enabling synthetic superiority without direct warfare. There may be initial variance, but the natural selection of the new biomechanical order will ensure that over time, species become more and more synthetic, with only rudimentary internal organs remaining organic.

Attached is an example of the process. To the left is Tali'Zorah vas Normandy, who survived the final push against the Reapers and was among the first generation of synthetic-organic life. To the right is her direct descendant some 10,000 years later, one Gali'Zorah vas Matanui.

I hope all of you who chose Synthesis understand what you've done.