If you're going to go with that strict of a definition of a serial, then I'd question whether they really have caught on in live-action TV, since I don't think there are that many shows other than
24,
Lost, and
Heroes that have a single season-long story arc and don't ever digress from it (I'm taking your word for it on
Lost and
Heroes, since I don't watch either). What other live-action shows do you think qualify as serials under this definition? I would have called
The West Wing, the new
Battlestar Galactica, or this season of
The Unit serials, but I don't think any of them would really qualify because they all have their filler/digression episodes or episodes where the main plot only inched forwards and the rest of the episode was largely irrelevant.
I would also say that
The Spectacular Spider-Man and
Avatar are still serials by your definition, although the first half of
Avatar's first season wasn't so tightly coupled and the first half of the third season was a bit padded.
Avatar might not have been an exceptionally well done serial by the end, but it was definitely intended to be one.
In any event, I think the obstacle to doing something like
24 in the past was that before VCRs, 500 cable TV channels, and TV on DVD and Internet video, you were dependent on seeing the episode when it aired the first time and there was no easy way to catch up if you missed it. Between all those technologies I mentioned, this is no longer true. It's easier than ever to watch an episode of a show and catch-up on what you missed if you like it. In a sense, I think more serial-ish fiction (if not proper serial fiction as you define it) may be becoming more common because it's an incentive for people to buy the DVDs. If something is a true serial, then people will want to watch it again to see if things line up properly and pick out all the little minutiae.
However, as I mentioned, I think the way animated series are made makes it a lot harder for them to be as tightly serialized as something like
24. You have to work on 2-3 episodes at a time or else you're going to spend a lot of time twiddling your thumbs, but that also means that if episode 6 gets delayed for any reason, then you're also stuck delaying episode 7 and 8, and probably every episode after that. A network is then faced with a scheduling nightmare or a much larger outlay (and risk) to do all the episodes up front and not schedule them until they're almost all done. Of the 2 serials you mentioned,
Invasion America was a mini-series (and, judging by
the way it aired, was indeed finished before it started airing) and
WITCH was an import, meaning Disney bought the show a season at a time. Don't ask me how the folks who did
WITCH did it, though.
-- Ed