I'm not sure what you mean. There has always been bad games, Steam just gives them a storefront. The same can be applied to Early Access: some of it's rubbish, some of it's great. Do people really buy whatever is suggested to them?
Its a
digital storefront. Its important to understand the key differences.
- Product placement. The store
decides what route you take through it to browse items.
- Data. The store can directly respond to purchase behaviour, a physical store does not.
Steam recently had a little problem with how prominently indie devs got shown on Steam. Steam has a continuous problem with the amount of content on offer vs the attention span of the visitors. It has enough shelves, right? So the
only limiting factor for Steam is exactly its userbase, its attention span, or put differently, for how long Steam can keep itself relevant.
Greenlight and the other programs are a responsive sort of thing; the audience is clearly open to buying into promises, so Valve gave people promises. You're right, there have always been bad games. But stores did a great job filtering the steaming piles of crap away for the most part. So it definitely matters when Steam starts a program like this. They increase the exposure of shitty games, and with it, the market share of it, and with that, shitty devs are kept in business.
Customers 'choose' but beyond the choice they can see, all they have is the illusion of choice on any digital storefront.
A practical example; if you're going through your daily highlights/suggestion list on Steam; if you get 9 indie games that are seriously crap and the last choice you get is another indie game that's a little less crap, what do you think might happen?