I've said it before, but I've never bought Jason as Dave, I just didn't, for the reasons you've stated and more - remember, he only took the role because he learned it was previously offered to his idol, Bill Murray. As much as I love Bill Murray, I'm not entirely sure he could pull Dave off either. Jason as Dave feels like Ross Sr. as Dave but with a chip on his shoulder (no pun intended).
I agree, the new series is doing much better, but it's largely due in part to Bagdasarian having more creative control: the character sound, act, and feel more like themselves as opposed to the characterizations in the movies.
On an unrelated note, your comment about each of the sequels becoming increasingly more kiddy makes me think of Shrek: the first came out during a time when DreamWorks was still Pixar's biggest rival, and their movies always tended to be considerably edgier than Pixar's; the first Shrek movie had swearing, tons of innuendoes and double-entrendres - the sequels to follow had virtually no language, and the adult jokes were toned down considerably (though the "Ye Olde Hooters" joke in #3 got the biggest laugh I ever heard in a theater . . . even though the little girl sitting next to me didn't understand why, lol). Ice Age's sequels are clearly Blue Sky's attempt to have a cash-cow franchise of their own like DW has with Shrek: the first was fine as a one-shot, the second sucked, the third came out the same time as that awful LAND OF THE LOST movie and made it feel like a dueling movie.