The geth were already sentient. At least as a network.
Agreed, but their networking capabilities were severely limited and they sacrificed mobility to reach higher levels of intelligence. 11 million programs is not enough to create sentience - much more is needed.
Also, each geth gets to become sentient? Umm, how many geth were needed to run a platform before? So, um, who gets to be in the driver's seat now?
There is a minimum number of geth programs for the platform's operation. With Reaper code, performance of those programs is boosted so the geth platform becomes a standalone unit capable of independent thought and analysis. I would even go further and claim that the code allows the geth to use Reaper capabilities of long-range communication, essentially bringing more platforms together and crearing sentience in a "traditional geth sense". Each geth platform thus becomes "sentient" in geth sense.
This is stuff the games have told us.
You are the one to decide how to interpret that stuff. You seem to have chosen to ignore ME3 events dismissing them as
Bioware butchering the geth and synthetics in general to have thie "bittersweet" endings.
Your choice.
I prefer following a theory that includes evidence from all three games.