So what is it that you think is going to make a web app perform poorly on these cheap servers? They're slow, but they're nowhere near slow enough that the time taken to render HTML for a realistic page on one of these cores is going to be a bottleneck. Each individual core has poor throughput, but there are a lot of cores. Doing a bunch of backend calls in series for a single page will make your webapp slow but that's always true, don't do that (likewise microservices). If you're doing heavy compute for a single request then yes your system will perform poorly on these servers, but that's not usual for a web workload.
"one core will execute the part up until the call to the backend and then a different core may well pick up the continuation when the result comes back"
This surely helps for one request if your backend or all your microservices are all on one machine, and you have several cores. But if you have your microservices on different machines, multi core will not help you speed up one request if it does not break down rendering of a page in chunks and distribute them to cores (and e.g. combine them with something like Facebook BigPipe (2010 tech)).
And yes multiple cores help with SEDA architectures but request and url parsing (which might be a SEDA stage) is too fast to have any real impact.
[1] http://codemonkeyism.com/a-little-guide-on-using-futures-for...