On serverless, I can enter numbers in a calculator and guess that running my little toy demo app on AWS will cost between $1 and $100. Getting hit with a huge $1000 bill and a refusal to refund the charges (and revocation of my Prime account and a lifetime ban from AWS and cancellation of any other services I might otherwise run there) would be totally possible, but I have zero control over that. Expecting to go on social media begging for a refund is not a plan, it's evidence of a broken system - kinda like those "heartwarming" posts about poor people starting a GoFundMe so their child can afford cancer treatment. No, that's awful, can we just be sensible instead?
If a server would have cost me $20 at a VPS provider to keep a machine online 24/7 that was at 1% utilization most of the time and was terribly laggy or crashed when it went viral, that's what $20 buys you.
But, you say, analysis of acttual traffic says that serverless would only cost me $10 including scaling for the spike, in which case that's a fantastic deal. Half price! Or maybe it would be $100, 5x the price. I have no way of knowing in advance.
It's just not worth the risk.
