St Helena is just a very isolated village and thus will never have mass tourism or much in the way of scheduled service.
Tourism will have to be very expensive to make it viable so rich people who want to see where Napolean died or go somewhere none of their friends have been will be the target.
I don't see them wanting to spend 5 hours ina ancient 733 configured for SAF internals .
But this sort of tourist is also the market sector covered by the specialised hi value hi cost organised 'world tours' where you visit eight or nine diverse and special locations over two or three weeks on a dedicated plane. The mighty 75 is a favourite for this kind of trip and they it seems can operate safely out of SHL so I can certainly see opportunities for addressing this top end market as an interesting stop over for a couple of days on a bespoke trip.
While I am well aware that Ascension is a military/spooky location and cannot be filed as an alternate it has I believe been used as such -Dl777 JNB-ATL so tis there if its really needed,
Its a heck of a lot of money though to build any form of acceptable airfield for such a tiny tiny market
PB