The "surround Lebron with shooters and versatile defenders" roster was literally ONE year, 2013.
In 2011 the Heat had almost no shooters and started a PG who couldn't shoot OR play defense.
In 2012 the Heat brought Battier in, which was huge, but they still had unreliable shooting at both guard positions and had no clue what they were doing at center.
In 2013 the Heat added Ray and Birdman and finally had a bench that could shoot and defend, though it was one of the oldest in the league. Moving Bosh to center finally fixed that issue though the Bosh/Battier frontcourt could sometimes lead to some rebounding issues.
In 2014 it was like the whole team got

at the same time, with Ray, Rashard, Birdman, Battier, Miller, JJ, Haslem, and Wade ALL looking aged at 33-38 years old or battling major injuries.
And the solution by the front office was...Greg Oden and Michael Beasley.
They regressed so much that there was one Finals game where they STARTED 38yo Ray and 34yo Rashard at the same time.
What do you think at that point if you're Lebron? In 4 years the team hasn't added a single decent point guard, their best center acquisition was a 35yo Birdman that nobody else wanted, Wade is only playing 50 games and looking like his career is well onto the downhill, and half the team is in their mid-to-late 30s.
Landing a gem like Whiteside was unpredictable luck, and it would have been offset by Bosh's issues anyway. Landing Dragic wouldn't have happened if Lebron hadn't left. In what way could he really have expected the team to get better, when they had just spent the entire summer of 2013 and 2013-14 season looking incapable of making a helpful move?