It's really hard to do something like that. We have prices from our upstream suppliers that we are bound to, and this makes it difficult to run a promotion permanently. So we're trying to keep the promo alive as long as we can afford to do to. The free data we give out amounts to millions of rands of value that we cater for by not advertising on TV or major print media.
If it was possible to make it permanent, we definitely would. But you can lock in Double Data permanently by joining Afrihost Plus+. It is an additional fee of R99pm, which helps us with the costs, but for most clients that's a lot less than what they receive back in value in terms of free data, Simfy access and free 500MB Mobile Data.
Hope that answers your question :)