I think much of this depends upon sales.
It is certyainly possible for most armies to have suppliments: chaos sects, eldar craft worlds, dark eldar sects, ork tribes, IG regiments etc.... This means that most players could potentially buy them and create a far more fluffy army that tailors to their specific need. This benefits the consumer.
That said, the thought of paying another £30 is quite off putting. Given that is the cost of the actual eldar codex there had beeter be some pretty decent rules, fluff, pictures, tutorials etc... in them to justify that kind of price. Especially if you do need the original codex too.
I suspect that what we are seeing is potentially the start of something quite large here. When you consider that (I think) all the codex's will be "6th" by June next year, that leaves a large release gap. Are they going to smash out fantasy? Perhaps a new game? All of these are unlikely I think given theyt are ignoring the specialist games. Additions like this allows all the codex's to be current but models to be released.
For example imagine if white scars are a suppliment. They will likely be able to release specific white scar bikes (might even be a limited release to keep down production costs), their special character, specific codex and maybe something else. Each white scar player wiull likely be spending £100 on average. Then add iron fists, salamaders, crimson fists etc... and that is just the marine ones!
In short, I think if this sels well we can expect more. If this sells rubbish then perhaps not. IMO it is likely to sell well initially but then really flat line as it will only be available on ipad. Curious though I am I have no intention of spending money on an ipad, Codex: eldar, Suppliment Eldar and perhapos the rule book......and I doubt I am alone in that.