It's always nice to read an optimistic spin but the "heart of a Champion" factor is wildly overrated: in the last decade, only teams with a prime Shaq won back-to-back titles (and in the previous one it was Jordan and Hakeem).
This team has a decent chance of repeating if healthy because they're good, not because of what they've done last season. They're not as dominant as last season because they've slightly regressed (especially defensively) while other teams have improved, but being as dominant as last season isnt' a sine qua non condition to win the championship.
I don't think they have regressed defensively, they're just not as focused as last year, which explains that they are less dominant because they are not playing 100% every night. When they had to step up defensively they did so this season but I acknowledge that they seem to rely on their offensive game a little bit more (and they should sometimes considering how dangerous they are in this area).
Sorry, but it doesn't really matter if you think they regressed or not, it's a matter of fact; your thoughts don't change what objective metrics say. What you're trying to argue is why they regressed, not if they regressed.
Do you have any evidence that the regression is due to that factor and, more importantly, that the team can "turn it on" for the playoffs? Generally, it doesn't happen that way. Teams increase their intensity and focus during the playoffs but so do their competitors: that's why the efficiency margin during the regular season is such a robust predictor of playoff success.
For example, Lakers fans use a very similar argument to yours: they argue their team proved that, when it matters, when they needed it, they stepped up on defence and that most of the season they've been coasting - and once playoffs start, they can get back to the kind of defence they played in the first months of the season and during the "statement games".
IMO, the explanation is more simple: Garnett is older and played less, Pierce has been tired, Rondo has been up and down defensively and we replaced minutes used by good defensive players with players who are not so good on that side of the floor.