Maybe, but Luke's lived in that town his whole life, and everyone says that he's always been the same. Plus, now correct me f I'm wrong, as I'm a little rusty, but wouldn't the time he'd been with Anna, have coordinated with the time he was with Rachel? Cuz he was with her for ages.
I see your point that Luke was always "the same", but i'm only saying it's possible because in his early adulthood he might have been more prone to being careless. That's as far as i'm taking it and i still put it down as a "jumping the shark" moment, no question about the shark!

I'm fuzzy on the time-line with Rachel and Anna. If Luke was somewhere in his late '30s when April appeared, and April was 14 i think, that puts Luke during his Anna phase at around age 23-24. I never had the impression they were a serious long-term couple. But when Sookie was telling Lorelai about Rachel she said they were together very early on when Lorelai was in Star's Hollow, because Sookie commented that Lorelai was busy trying to survive with little Rory and would not have been aware of their relationship. Luke & Lorelai are about the same age, so it would have meant that Luke was with Rachel in his late teens to early twenties, before Anna. I suppose it is possible he was with Rachel both before and after Anna; Luke's issue with Rachel was that she had a tendency to take off at a moment's notice, so maybe they were together as a couple for different periods. And in between one of those times when Rachel was out of the picture, rather than hibernate in his room, Luke had a time with Anna.
Oy with the time-line already! I'm getting dizzy. Too dizzy to jump a shark.

It is all very fuzzy and i think that's because ASP did not even consider the April story line until long after she had created the Rachel story line.