> Once you hit high levels of traffic, instead of panicking and trying to add more servers and fixing DB structure, you do a cost benefit analysis and see what to do next. Either work on reducing your cost slope, or make it logarithmic by moving to regular servers.

...and that's a good reason to try to avoid the lock-in of AWS specific services. Relatively entry level setups on alternative platforms will let you scale well past the system where AWS costs starts to get painful and people start panicking about how to cut costs. When you hit that point you really don't want to be facing a major re-engineering effort to be able to even use other options to bring the cost down just as the costs are soaring.