I'm not sure the server side is much better. Complex UI is complex wherever you try to implement it. You can say that is not actually possible to create really

I'm not sure the server side is much better. Complex UI is complex wherever you try to implement it. You can say that is not actually possible to create really smooth UI with animations only using server side code. And server side code usually don't want to keep any state at all (to allow the use of multiple servers) which causes a lot of problems. A SPA can keep state and use it if it suits the problem better. Some of the new js libraries actually looks more developed than most server side ones. [edit: spelling and a clarification]