Wow, your second statement is really not fair at all. Steve Nash is 36 years old, 10 years older than both Lebron or Carmelo. That's the only number that really matters. It has little to to do with how long any player has been with their respective team.
The Suns also won something like 15 less games last year than they did a few years ago. Their decline in wins has fluctuated a bit, but looking at the statistics, you can't argue that they haven't declined. They also have $81 million tied up into three mediocre players for the next several years.
I don't claim to be an authority on these things but that's my perspective. I don't really care either way about Lebron leaving (I just hate the way he did it) and Carmelo is barely a superstar that merits this sort of consideration, but again this is all my opinion and I think Nash is fine to seek greener pastures. I just think your suggestion that there are no evident metrics is not correct.