They haven't lost so much motivation. They are playing hard. I think Bosh
himself is in a bit of a momentary lull, but it's just part of going through the marathon. I don't think the concept of positionless basketball is working over the course of 48 minutes, if it means Lebron needs to defend mostly bigger players over that time.
This team tried a ton of different lineups last season. Nothing seemed all that satisfactory from one week to the next. Even in the playoffs they went through all kinds of change-ups. Lebron gives them a lot of different ways to win, since he can play so well at 3 or 4 positions. But as a team they kill other teams defensively when he can guard small forwards and guards. When he has to guard bigs they can shoot over him regardless of how well he defends, and Wade is exposed more against the smalls at the same time. But if they get really good at incorporating different options throughout games and against different matchups, then they will be far ahead of last season where they were really just winging it so often.