I think it's reasonable to expect 2 APs a year, but I will point out that a storyline is not the same thing as an AP. The reporter asked how often we can expect new storylines. Tyranny of Dragons, which is the specific storyline referenced in the interview, was supported by several different products including expansions to the Neverwinter MMO, minis, DM screens, etc. as well as HotDQ and RoT books which together formed the AP for that story.
There is plenty of room in Mearls statements left for Wizards to bring out 2 storylines a year which are supported by several products, but do not include an AP book in the mix. For example, I think it might be reasonable for them to release a campaign setting and a small module that covered a couple levels in support of a storyline. Mearls statement would still be true, even if there weren't a big AP for that season.
Releasing an AP for the first two storylines does set a strong precedent and establishes certain expectations, but that doesn't necessarily guarantee that the next storyline will be released exactly the same way. I suppose some will still complain that they didn't get a new AP as "promised" but I'm willing to give Wizards some latitude to roll out storylines with a different mix of products in the future.
In light of recent statements by Chris Perkins about trying to shake up expectations and surprise people with the way that setting material is released, I don't think we should jump to the immediate conclusion that every storyline will have a big AP book that covers levels 1-15.