I think it is different enough that it will have an effect.
Currently, at least in console terms, the buying choice you have is between a brand new sealed game at £35 or a second hand, slightly tatty game more than likely with a scratched disc at £30.
For the vast majority of people, saving that fiver isn't worth the trade off and so the new market doesn't suffer particularly drastically.
Move to digital only and suddenly you're looking at £35 for a 'brand new' digital license, which is in effect, immaculate versus £30 for a 'second hand' digital license which is in effect, also immaculate. Who in their right mind would pay £35 for something they can get for £30 and is absolutely identical in every single respect possible except for where your £35 or £30 goes?
It's a situation we can only speculate on because we've never before seen a market operate where a new market and second hand market offer identical items for differing prices. Simple economic logic dictates that the cheaper product will be more attractive, thus attract more demand and so have a much larger effect than we've seen before.
Plus, you can sell a key by sending an email, vastly easier than selling a physical item and so you don't end up in the situation where your selling price is more like £10 to the local GAME store because selling yourself is more hassle than it's worth. Suddenly you can easily attain £30 for your second hand game, not £10 or £15, so the supply of available second hand games will be much higher than ever before too.