This line of thinking probably isn't much different among FO counterparts around the league. He is in charge of maintaining the team, and he's trying to do that. Self-serving business.
The situation at hand is very different from the situations each of the Big 3 faced four years ago. Wade,Bosh, and LeBron, from what I understand, discussed playing together since the Olympics and the Heat gutted their roster in order to make sure that could happen. The possibility of splitting up after 4 years(and 4 NBA finals appearances) doesn't seem absurd, but it does feel like they need to give this more time.
However, this is more about LeBron than Wade or Bosh. Wade and Bosh will probably stay put in Miami, I would be shocked if either left. LeBron seems to be the one on the fence. For him, I think he knows that there is a possibility of one or two more championships, but he also knows that Miami in the coming years will be in decay with Wade showing his age,Allen probably gone, and Bosh getting older as well. If he signs with Miami, he might be committing himself longer than he wants. He does have another year to decide if he wants to wait out for the player option in 2015-2016.
Point is, Riley is trying to keep the team together and he's worried LeBron,specifically, might bolt. He didn't give a shit that LeBron and Bosh were leaving their respective teams to come to Miami, I doubt many FO guys in his situation would. Even if he did care, like I said, the situations are obviously different.