[garethmcc]

There are an enormous number of people using Lambda's as a part of very complex workloads including customer facing platforms. Your specific use case of 100 000 incoming requests in 20 seconds is actually far more extreme than most users use cases; Lambda can do 3000 instantly and then adds more capacity over time unless you contact AWS and ask them to increase the limits in which it can do a lot more.

There are very many users of Lambda that run very high volume production workloads (billions of requests a month) and save an enormous amount of money doing so compared to traditional techniques. A lot of that saving comes from the ability to create solutions in days as opposed to weeks or even months and the fact that for most use cases the capacity exists to handle the volume most users need.