No, 2.1 will not be an upgrade like 2.0.12 to 2.0.13 was, you will need to start over especially because themes will need to be redone for 2.1 to cope with all the responsive changes. It took me a long time to port one of DzinerStudios' themes to 2.1 and I still have to do work on it occasionally with the 2.1 changes.
But it's been possible for some time to limit theme edits and you'd be surprised what can be done without theme edits depending on how far you're willing to go.
It's also possible to do truly insane things like create hooks where none exist and hang things on them - without file edits, but that's really getting into the realms of the behaviour that most people shouldn't attempt without truly good reason.
Also to clarify, 2.0.x is only receiving security/maintenance patches, this is not a maintenance improvement.