I don't think Tim and DF wanted to cater to the casual/mainstream audience necessarily with Broken Age. Rather, I think it's more a case of "re-evaluating" adventure games. Kinda like Josh Sawyer, you know, trying to "fix" the genre so to speak.
I think a lot of adventure game developers live with this stigma in their head from the 90s and present days when they were/are criticized for pixel hunting, nonsensical puzzles and solutions, lots of backtracking and so on. As a "game", the adventure genre is pretty flawed. Don't get me wrong, I love it, but I can see why it faded to obscurity back in the day.
Make no mistake, that was the path of LucasArts adventure games as well. First they removed the Sierra parser because it didn't add much (at least that was the though-process). Then they removed dead-ends and game over screens. Then Tim himself tried to put more "interactivity" with those bike action sequences in FT and the tank controls in GF.
Mix that thought of trying to "change" how adventure games are made with the lack of experience of doing proper puzzles after so many years since Grim Fandango and an unnecessary focus on pwetty graphics and star voice acting and you get BA.
In other words, I don't think they tried to intentionally "dumb it down" as much as this was their idea of doing an adventure game "properly".
People tend to mix the situations or try to put it politely. Broken Age isn't a case of catering to casuals or to little kids. Bullshit, I played Monkey Island 2 when I was 6 and I absolutely loved it, even though I didn't understand what they were even saying at that point. Kids have the patience and the ability to explore and try all sorts of combinations for solving puzzles. It was like playing in a cartoon or story book.
It's the adults with their jobs and problems who want a little nostalgia of the old times through quick gratification, those are the problem. The "mature" audience and the developers who stopped treating games as toys and want to treat them as something else.
Broken Age is not a case of calculated cynicism, of trying to sell to the invisible iPhone audience. No, it was just pure incompetence. A bunch of hipsters not being capable of making proper adventure games, preferring to masturbate each-other thinking of ways to "revolutionize the formula"
fucking hipsters