I would agree the story has a much lighter sider than movies previous. Raiders was the the straight-edge adventure epic, but still bright and powerful. TOD throws you for a loop with pulpy/dark/drama while maintaining the character traits and style we're familiar with. LC was a bit of a band-aid for the series after TOD left parents and the MPAA shaking their heads. This means many people believe it fell into Return of the Jedi Syndrome, repackaging the first movie with what audiences liked, add a few twists and make it a suitable climax (Obviously this wasn't a story arc like Star Wars, but it had a similar dramatic pattern).
I still think that the strongest part of LC's story was in fact the Henry Snr./Jr. subplot. It explains the nature of Indy a little better, his preferences for fieldwork rather than study, this self denouncement of his father's name (thus Indiana after his dog, someone who showed him much more love and positivity than his father. He took the name of someone loved him, and it wasn't his father's).
This is the overbearing theme of the story IMO, the search for the Grail is what finally brings them back together. Notice at the beginning of LC he wants almost nothing to do with his father, but after rekindling their father/son relationship they learn to love each other as a family, a family that was broken after Indy's mother died and Indy rebelled. Further evidence is his genuine reflection and sadness after his Dad has passed in KOTCS.
I think the feelings would have been more of relief if he had not had his new experiences and adventures with his father.
Last Crusade's story ends up with this great overbearing sub-plot, but gets washed over by recycled action and drama that in a way yes, shows signs of effort slipping out of the franchise.
Well I guess I just convinced myself I like LC after all, but even still My Fav's are Raiders and TOD.
Luvey-duvey? I suppose it could be degraded to that, but only in the last few minutes when Father and Son finally accept each other, but I believe there's more to it than that.