There should not be a bitcoin successor.  Other than minor changes to the blockchain, nothing can be improved upon from the view point of the consumer. Satoshi planned all of bitcoin's  features ahead of time, and it is his design that made bitcoin a success. If there is a successor to bitcoin, it would be something issued by governments, as where each country would have their own digital currency.  The latter is probably a bad idea, therefore bitcoin should be around for a long time to come, at least in the western world, where if it came down to it, courts would enforce its existence.