I'm not sure everyone posting hear appreciates the situation in the summer. It's easy to lookup the bus and train times and conclude there are alternatives. They reality on the ground is somewhat different.
First to the number 50 bus. This uses the Sandbanks ferry. This normally runs every 20 minutes and in summer the bus every 30 minutes (with the same running time for both departures per hour). It doesn't take much to work out that you're not going to get both buses to Sandbanks to board the ferry without a wait when the ferry runs at a different frequency to the buses. However the ferry doesn't always run to timetable. When it gets busy and queues are forming the ferry runs continually (I.E. it doesn't wait for a specific time to depart) to try to clear the queues. So this means even if the buses are on time there is no guarantee they will get there and find the ferry is there, it might just be leaving and hence a good wait for the next one. On the Sandbanks side there is some bus priority in that the road around the peninsula is one-way with one lane for through traffic and one lane for the ferry. The bus uses the "through traffic lane" and then has a short bus lane to cut to the front of the ferry queue. However this too can break down when the queue for the ferry gets beyond the one-way road and then blocks up the road for all traffic. This happened a number of times last year and on several occasions the police actually closed the road (though I think it was open for residents and possibly buses) as the roads had essentially become gridlocked with traffic queuing for the ferry and car parks.
At the Studland side there isn't any sort of bus priority. However the bus does have flashing lights and a siren on and when they reach the end of the ferry queue I have seen them put this on and drive on the wrong side of the road to get to the front of the ferry queue. This strikes me as dangerous and I've seen conflicts with drivers pulling out of the boat club or car park and finding a bus driving towards them on the wrong side of the road. It seems very dangerous to me. The road isn't wide enough to put a bus lane in and keep it open to two-way traffic and I doubt the National Trust, who own the land, would agree to it being widened.
It is also horribly over-subscribed in the summer. It is common for buses to leave the square in Bournemouth full and leave people behind. There have been fights breaking out in recent years (yes, really). It did used to run every 20 minutes in peak summer, but that hasn't been the case for a good few years now. I really do feel that More Bus could do with some competition on this route. I can personally attest to the bus refusing to stop due to being completely full a number of times in recent years. I have complained to the company but they don't really seem interested in doing anything about it and it's quite predictable if it's the summer holidays or a weekend and the weather is good that it will be too full and leave people behind.
So in summary this bus is horribly unreliable and I wouldn't want to rely on it to catch a specific train at Bournemouth.
As to the route 40 well again in the summer months the A351 is often queueing traffic all the way from the A35 junction (Bakers Arms roundabout) to Wareham, in both directions. It can easily add 30 minutes to the journey time and sometimes more. There are also often queues to get through Corfe Castle. Again, there is no bus lane and probably no room to add one (the Council did widen the pavement to create a cycle path but even if that went I don't think there would be room for a bus lane). In addition at the moment except on Saturdays, SWR have reduced the train service from Wareham to 1tph each way (it used to be 2tph). So going to Swanage, the hourly train arrives at Wareham at 27 minutes past the hour. The bus departs at 11 minutes past the hour. So you have a 45 minute wait from arriving at Wareham to getting on the bus to Swanage.
So I don't really think the bus is a good alternative, especially in the summer when they become so unreliable.
I do recall on the first year of the trial Swanage Railway did say that the passenger numbers exceeded expectation (and I used the service) so it disappointing that they still haven't run the 2nd year trial. I was very happy when it was re-connected up to the main line but I've been very disappointed that it seems the same every year, Swanage railway announce the intention to run the service at the start of the year but then later abandon the plans. This has happened for a good few years now.
I don't think many people would commute from Swanage to London but I do think there would be demand for a service for commuters to Bournemouth, Poole and perhaps Southampton, given the local congestion and that could be offered. If there was sensible planning maybe tickets could also be inter-available with the buses so if the times of the train (if they run) doesn't work both ways commuters wouldn't be forced to pay again for the bus.
Yes Swanage is a small town but there are really only two roads in and out, the A351 north and the route via the ferry, both of which are unreliable so an alternative route would I think be very welcome.