I disagree to an extent. If Ronaldinho decided to go to an MLS team, he'd still make the Brazilian team because he's already proven his credibility at his current age. Now if he decided to go to the U.S. at age 31 or above, I think he'd then be automatically shun out for the reasons you mentioned. Even Beckham at his age has a strong shot at getting England national team callups if he plays for the Galaxy the way he has for Real Madrid. He's been proving in Spain that at his current age, he's still one of the best in England. There's no way England would be stupid enough to shun him out of a team just because of the team he's playing for if he proves he can contribute.
Didn't Dunga played in Japan? A league which isn't that well known or that isn't stronger than South America? Yet Dunga went to the World Cup. Dunga hasn't called Ronaldo yet, or might never call him. But Ronaldiho presence in MLS won't hurt his chances with Brazilian national team, he's already their star, without Ronaldiho could lower the value Brazil's price. Dunga wouldn't cut Ronaldiho for being in MLS.