As a third party vendor I don't think Steam have the right to stipulate a SLA agreement like that.
Under normal circumstances, I would agree with you, but early access is a far cry from a regular retail release. The rules are a little more vague. I don't see it as "pre-order with a bonus", developers get a huge benefit from early access systems, both in funds and in feedback. I think 1 year is more than generous for a game to be in early access (maybe 18 months on the outside), and any longer than that is just milking it. If the developer can't get it to the stage where a year is sufficient to finish development on their own, then they should look elsewhere for the funds they need.