You can publish/consume webservices in sandboxes just fine. We don't advise people to publish webservices though, as the sandboxes go to sleep.
I don't completely understand what you mean by "old dev environments"? We're not replacing anything with sandboxes, we're introducing them. Previously, if you had a Mendix project without a connected node, you wouldn't be able to deploy anything. Sandboxes now makes it possible for you to always deploy, regardless of whether you have a licensed cloud node from mendix or not.
In the old days you would develop on your local machine and you could start the model local. It would shut down after two hours. You needed a paid node in the cloud to deploy your application. Now with the sandbox anybody can deploy to the cloud. Now after an hour of inactivity (so no users in the environment) this sandbox node will shut down and go to sleep. If you go to the URL again the sandbox will reactivate itself and your application can be used. The only limitations are the ones in your other post. Only 10 active users and 100mb diskspace etc. But for the rest you have a working application just like you would have if you paid for the node.