When I was a triathlete, I trained some with an Olympic 1500m swimmer who swam mid-15:00s for 1500. Not mediocre.
One of the best triathletes in my area just ran 2:21:xx at Chicago and got his Provisional Qualifier for the Trials. Not mediocre.
Chann McRae, a pro IM, and Steve Larsen, a former pro IM, both raced bikes in Europe, as did several other famous pros like Chuckie V. and Ned Overend. Not mediocre at all.
I don't buy the argument that triathletes are necessarily mediocre athletes. As for AG triathletes: there are more slow and mediocre runners out there than there are mediocre triathletes. And good for them--it's beats the shit out of sitting on the couch and dying a slow death.
If you're trying to race an Olympic triathlon, you are pegged for 2 hours--pegged. Try swimming 1500m in 20 minutes, cycling at 25+ mph, and running sub-34:00 for 10K, all in the same race. I've done that, and I wasn't even very good at Olympic distance racing--there are many who are much, much better.
I don't do the sport anymore because I got tired of it. I'll get tired of running sooner or later and race bikes.