You know what most people do when they finish with a console game and don't play it any more? They trade it in or they give it a to a friend. You know what those same people will be doing in the next-gen when Microsoft (and maybe Sony if the publisher's decide) blocks used game sales and game lending? They'll bin it. That means that sooner or later, there will be millions and millions of games in landfill around the country.
Or they could burn them, which just releases all the harmful toxins into the air instead of the soil.
According to this piece over at Kitguru, the current used game market sees well over 100 million discs sold each year. While the Xbox One is unlikely to have such an audience in the early days of its life, once it's established it could very well have a market that approaches that size - or it would if it allowed used games.
Instead those discs will be languishing in people's houses until they decide they don't need them anymore and throw them away.
In the past, games have just passed from house to house as people bought and sold games from the current and previous generations and eventually they would end up in retro stores. Now that won't happen as they'll have no resale value other than as collector's items, which won't happen for at least 15 years or so.
When it comes to the next gen, we may all need to start looking at disc recycling.