Prepare for the average and hope for the best seems like a great way for things to fail.
I didn't follow /r/place that much, but I haven't read any complaints about latency or failures so it looks like they did just fine.
Edit: you're an sre, you probably have more experience planning these things than I ever will. I just can't help but think about what would happen if some trolls realized they could synchronize their updates and bring down the service. (Although based on the infrastructure it doesn't seem possible to cause lasting damage.)