They do this because they don't have the resources, or in some cases possibly the functionality, to build completely new things from the ground up.

I don't see how adding high level mobs to old areas changes anything. I don't find anything terribly confusing or complicated about having a seperate area for high end content.

New currencies are made because they want you to start from scratch on new content. It's the same reason FFXIV changes out the tomestones in major patches. So that you can't bank them beforehand and then not have to do the new content, it also places a burden on players of wondering whether they need to spend their old currency or whether they should spam that content before a patch comes out to get the new stuff immediately. It defeats the purpose of adding/updating the content in the first place if people don't have to do it.