The reason I say this is first of all based on the evidence. We already see data that shows that higher IQ's in human beings are linked to lower reproductive rates. That's a scientific fact. Lower reproductive rates are typically not beneficial from an evolutionary standpoint.
With Intelligence comes the ability to interfere with reproduction (and natural selection) and when a species begins to substitute its own judgement for natural selection, it seems likely to end in catastrophic disaster. Examples of this would be the invention of medical technology which allows people who are genetically weak to survive and propagate... the creation of extremely toxic chemicals. Pollution. Abortion, which substitutes human judgement to determine which offspring live, rather than genetic fitness.
The ability to interrupt feedback mechanisms that regulate population (such as starvation, pollution, etc) and create very dangerous imbalances that could wipe out the entire species is something that typically is feasible due to intelligence.
For all of these reasons it seems to me that intelligence is probably an aberration that springs up, survives for a few hundred thousand years and probably ends up in extinction of the species, or is bred out of existence.. disappearing like the dinosaurs.