pkl

No business or web site which has 10K concurrent users will be using just one server. Scalability beyond 10K users/server doesn't buy you anything.

I have and if you use blocking IO or NIO its like twenty lines, code just about any one can read and maintain. Why make it more complicated than it needs to be?

You can handle a much larger number of connections provided they don't do anything useful. However if you expect to do anything useful, you are likely to need some horizontal scalability once you have thousands of connections.

AdWords charge about $1 per click. Lets say your hardware budget is only $1 per connection. With 10K connections you still have enough money for 10 servers if you wanted.