Taking daily pageviews and dividing it by 24 * 60 * 60 assumes your traffic curve isn't a curve at all - it's flat. For your average US-based consumer web app it's highly unlikely to have just as many people using your app at 3am as you do 3pm.
When scaling a web app, you need to prepare for peak usage, not average usage.
Here are a few first-hand examples. Friends for Sale, a social game, had ~10M daily pageviews (100 req/sec avg) but had peak load of > 200 req/sec. Ideeli, a "flash sale" designer shopping site, typically had peak load 5x-20x higher than median load.
Regardless of what Quora's traffic curve looks like, wheels' point still stand - it doesn't matter what backend you're using for 60k users on a web app.
Peak on a decent traffic site will be roughly 2x-3x the average. 5x-20x would be a blog that gets 10 visitors a day and after being posted on news.yc gets 100 visitors.
There's an entire branch of mathematics that covers this. It's called "queuing theory". It's pretty strait forward to find the probability of N requests per second for an M/M/1 system. Then you decide, "I want to be able to handle the 99.9% case" (or whatever) and you know what your peak capacity has to be.
Your point is valid, but it doesn't work like that.