If you only play 2 AAA games a year, or just play the little indie things a discless xbo might be okay. But thats not the majority of people buying a new console. For most people there are 3 BIG problems with a diskless system.
1 download time. 12 hrs to several days to download 40+ gbs next gen games.
2 harddrive space, the only thing worse then having to download the big games, is having to delete them to make space. At launch there were already too many games on the xbo to fit on the 500 gb harddrive. 2 years from now, when there are a ton of games and they are larger than 40gb even 1tb is going to be small.
3 internet connections. Not just the time it takes to download the game but the data usage will be a problem for some. Right now people in many places who are in the top data usage get throttled, thats just going to spread to more and more places as more and more people and devices connect to the web. And they are talking about straight capping the amount of data you can use in a month, like on the phones. If that happens who is going to be willing to download the games? With net neutrality gone this is increasingly likely to happen.