I think the whole "league more athletic" thing is a little overrated. A lot of those old teams played at a pretty fast pace. And there was always athletes in the league. Guys like Worthy, Nique, Jordan, Michael Ray Richardson... etc... those guys were ripped out. It surprises me a lot when I look back at videos or images of players from the 80s or 70s or even 60s/50s... its not like they were a bunch of slow fat white boys. Look at a guy like Oscar Robertson for example:

That guy is a beast, man. It's not like suddenly the game has evolved into a bunch of superhumans. It's always been full of peak athleticism.
The point is... Bird was considered "slow" and "unathletic" in his day and he had no trouble dominating. He'd have no trouble dominating today. And with the watered down league he'd probably dominate even more.
I'm not sure why so many people think the game has suddenly evolved. Bird was dominating in a time when Isiah, Jordan, Magic, Malone, Stockton, Nique, Hakeem etc were around... Jordan, Malone, Stockton, Hakeem were able to dominate in a time of Shaq, GP, Kemp, etc... Shaq was able to dominate in a time of Kobe, KG, Duncan, etc... I mean this is just a dumb list off the top of my head, but its not like we're talking about Bob Cousy trying to deal with an Iverson crossover.