The problem is that it would radically change the relationship between publishers, advertisers and users.
While I agree they (being Google) believe that, I am not quite sure that would be a problem. The industry is seeing issues on multiple levels that this could help with.
First, there does seem to be in the publisher space, way too much available space for the available inventory. Let the choir of "I don't make the kind of money I use to" people start on this one to show what I mean. Publisher side advertising has never been popular with advertisers. And yet it grows exponentially. Which results in lower CPM and unhappy publishers.
Second, it actively culls the visitor base that wasn't likely to click on ads anyway. Let's face it. The people who would pay $.99 to a site would likely have not clicked on ads anyway. So isn't it better to make something from them than nothing at all?
Second and a half, related to the above point, it now gives you a way to monetize people who would either like to use an ad blocker or would like to support specific sites while using an ad blocker. People use ad blockers because they don't have a choice. And when there are "choices" they are tedious and people forget to do them. It is why the $0.99 app model works so well. Nobody minds spending $0.99 and it automatically renews without you having to think about it.
Second and three quarters, again related, it would give low ad success platforms such as forums a way for members to easily support those sites in an ongoing fashion. Let's face it, some types of sites just don't do well with a traditional ad model. Forums leap to mind first, but I am sure there are more. I would lay the recent downfall of forums squarely at the feet of the fact that they are so hard to monetize. Sure, you can charge a subscription fee, but for the average web site owner, the fee you would have to charge to make it worthwhile is higher than most people want to pay. And the logging in and all that just confuses things for visitors and a confused mind says no. If AdSense had something like this, it would open up a whole new realm of people to get money from that they currently are not reaching.
Third, it would result in better ads because with the ad space inventory shored up, those bottom feeder ads like "Do this one thing..." would not be able to flood the market sucking up as many low CPM ad spots.
Fourth, It gives a publisher a reason to only use AdSense or to stay with AdSense. How many "I quit AdSense" posts do we see these days? That can't be a fluke of just WebmasterWorld. I suspect that publishers are leaving in droves. And maybe Google is happy to see them go because they don't fit well with the current AdSense model, but if they could offer a good subscription model, they might get those publishers back and be able to again generate an income off them. Then for publishers like myself, who use a combination of ad providers on the site, it would make it much more attractive to only display AdSense if I knew I could monetize with a subscription no-ad model as well.