First mover is a strong part of Valve's position: they have almost two decades of not failing their users under their belt, very few other digital content purchase platforms have a similar backlog. iTMS appeared a little earlier, but the rebrandings haven't exactly been helpful in terms of perceived seniority and it didn't really start with a wide audience, because non-pirated access to hl2 was much less of a rarity than non-pirated filling of ipods.

And as long as Valve does not burn that trust (there have been a few hickups in the very first years, but quickly forgotten), their lead can only grow. You can't throw money at the problem of potential customers wondering what the chances are that the service will still be around in five years. I'd even say that throwing money would only increase doubt: the more money invested, the higher the threshold of success required to be considered worthwhile of continuing.