I think the fundamental problem with reaching the Swanage area by public transport, as others have said, is the expense of the last bit: Bournemouth or Wareham to Swanage. So if you're travelling from say Southampton to Swanage, perhaps a third of the total fare is for the last bit into Swanage.
Through rail/bus tickets with the bus section charged at a more affordable rate would help; PlusBus would be a good idea, I agree. I've used PlusBus to reach Wilton (near Salisbury), to better access the South Wilts countryside compared to Salisbury itself, and found it good value.
This is the one factor that has put me off travelling to Purbeck by PT, it's a very expensive destination to reach compared to other picturesque parts of the south at similar distance. Compare south Hampshire to Arundel or Amberley by rail, which is considerably cheaper. Or even the highly underrated Tisbury area, a hidden gem and a still-unspoilt area of the central south easy, and not expensive, to reach by train. With a Network Railcard, even distant Eastbourne (for Beachy Head, Seven Sisters etc) from south Hampshire is cheaper! (or it was in 2013 at any rate..)
I guess, however, that the pricing is based on what enough people are prepared to pay, so not sure if it will ever change other than if a government-driven 'green' agenda is followed to price train-bus journeys more affordably to reduce the number of cars on Purbeck roads. In the absence of the SR operating, I also would support an hourly Wareham station-Swanage shuttle bus (not originating at Poole) which would be geared, and timed, for rail connections, routed via Wareham town, the A351, Corfe, then in alternate hours the direct (but less interesting) route, or the scenic route via Kingston, and Langton Matravers.
As for the SR operating through services - it's obviously a different problem but a return of those 159s (or even 158s) to Corfe in the summer season might be doable, if one unit is available.
The mistake I think was closing it in 1972! If it had never closed it could probably support a Bournemouth-Swanage shuttle nowadays, particularly (as others have said) it would provide additional services at intermediate stations. Could also have been electrified at the same time as Weymouth if kept open, which would have opened up other opportunities.