I agree, it is a daunting problem.
Another problem is that it's hard to experiment in the browser; the stakes are very high when bugs or poorly-conceived ideas quite literally break the internet for users. As a result, the development cycle is righteously slow (~18 weeks), the cost of landing code in Firefox is righteously high, and it's just a difficult environment to try new things.
On Test Pilot we aren't trying to build the solution, we're trying to find it. It's a different model of product development than we've historically had at Mozilla. In this case, side tabs are the start, but you can expect new features and concepts to start coming down the pipeline as we start gathering feedback and data about how people use them.
