It's strange how the fact that it has an easily rememberable IP address has driven adoption of 8.8.8.8. I have a couple of other DNS servers memorized from before 8.8.8.8 appeared, I'd likely switch to them.
I guess the predictable would happen. Badly configured systems would stop working. Systems with a working secondary DNS would keep working.
More interesting question is what could you do if 8.8.8.8 was compromised? That could be interesting.