B752 is hardly a museum piece. It's a very well built and extremely capable aircraft that is clearly the best option for the demanding nature of St Helena and the thin loads it is likely to have.
Until we get the A321LR, whose actual performance is to be proven, you can't really use anything else if you want to connect the island to Europe. From SA you have more choice, but clearly the B737 didn't cope too well,I doubt that a small Airbus would be any better.