Wouldn't they likely use the faster, (much)cheaper and (much) more frequent 50 bus instead?
Well my post was a light-hearted rumination on the fantasy that the Swanage line were part of the national network, electrified, had reasonable line speeds, and trains from Bournemouth to Swanage were as frequent as, say, Brockenhurst to Lymington Town.
So if we say that Bournemouth-Swanage would be equivalent to Bournemouth-Moreton (about the same distance) it would take about 39 minutes, cost about £13 for an Anytime return, and the first train might arrive at around 7am and the last depart after 11:30pm. Frequency half-hourly [I know Moreton is only hourly, let's take Dorchester South for that bit of the comparison, Swanage isn't going to have many trains passing through non-stop

] and good reliability and comfort, 250+ seats.
A very 'commutable' journey in other words.
The bus no 50 by comparison? Tickets £2 each way at the moment - but only whilst the subsidy lasts, then it will be £10 there and back. First bus arrives 09:05 - fat lot of good for a 9-5 job, even if it could be relied upon to arrive at anything like that time after a slog through rush-hour Bournemouth and a chain-ferry crossing! Journey time 70 minutes at best, often timetabled for 80. Frequency half-hourly, except when it isn't. Last bus back 22:05. Lively and bouncy journey (to put it in the kindest way possible), just don't expect to be able to use a laptop - even typing on your 'phone will be a challenge. But will you get a seat, or even be able to board?
Anyone trying to commute daily to work that way would have to be off their rocker.
When I was looking for the capacity of the Breezers, I didn't find the figure, but the first offering from Google was
this one - says it all really...
Advertised as the Purbeck Breezer, the 50 is managed by MoreBus using convertible EvoSeti's, running from Bournemouth Station-Swanage. It is timetabled to take around 70 minutes, but some serious rescheduling needs to be done before next summer, especially after my experience riding this route. It does provide some brilliant views, but definitely avoid the "tourist rush hour", as your journey could be painful!