As for any future installment's familiarability for newcomers to the franchise, isn't that the same for pretty much all serial games?
I don't think we can generalize like that. Half Life 2 was perfectly playable and understandable without knowing the events of Half Life 1, partly because there's a sizeable time lag between the two stories.
That's not the case in HL2 -> Ep. 1 -> Ep. 2. Things happen imediately after one another, so playing Ep2 without playing the others feels really awkward. Especially as they all end in huge cliffhangers.
Episodic content still makes sense IMO as long as they don't spend more than 1,5 - 2 years between episodes, or they risk releasing the games for different gamers generations and making people forget what happened before.
Episode 3 / HL3 has crossed that threshold way long ago, in my opinion.
People have crossed the "anticipation" period and are getting into the "I don't give a sh**" period.
At least for me, I'd have much preferred an Episode 3 in 2009 than Left for Dead 2, which was criticized to the bone for launching way too early. (BTW I purchased L4D2 and
never played the game, and I don't feel like playing the game any time soon TBH.. I'm just full of the whole Zombie movies+tv series+game histeria right now.)