I think the one point we are missing here is that Luca's death was depicted as "easy" to show us that times were changing, the Godfather was slipping, Luca wasn't as immortal as they all thought, I mean.....look at the scenes before he goes to meet with the Tattaliga's (???), they show him putting on his vest, how scary he looked walking down the hall.....his death showed us that the Corleons, like Luca, were not as smart as they thought, that there were some gaping holes in their armor. This made Michael's coming to the rescue so dramatic, and soo important, it showed the change of time and the passing of the torch.
Agree???