Define success for a party not in power?
In reality, it’s the stay at home Tory vote that will decide who wins the seat at the next GE.
If this was a GE Labour wouldn’t even have targeted Tamworth, it’s that far down the list of targets. There is an argument that in losing this, the Tories have hurt Labour in a minor way as they will now have to pour resources in to defend the seat at the GE (Unite will insist as the MP is one of theirs)
If the Tories don’t regain Tamworth in the GE my prediction of them being a spent force in politics becomes more likely.