Donovan would be a regular starter for the majority of the EPL
Exactly this. That's great ambition for Stu Holden, Brek Shea, Geoff Cameron... Maybe also Omar Gonzalez, Graham Zusi... who else?
Tell me why Donovan joining an EPL team is ambitious? He may not be a world-beater in absolute terms, but he's his nation's leading international scorer by a large margin. If you tell me his joining a major team to chase the Champions League, I'll agree that's ambitious. That's not the argument made by the previous commenter. Instead, the argument was that joining a mid-table EPL team shows no general ambition, but instead investing in the development of the MLS is much more ambitious. I agree with that argument.
The point on Tony Parker and the NBA is opportunity cost. EPL may be the best league, but there's also Serie A (is Michael Bradley not ambitious?), La Liga, and Bundesliga... where do I stop? An ambitious footballer has many choices, and the difference between one choice and the next is often very small. In basketball terms, there is the NBA. There are a lot of good Euro leagues, but none of these are even a reasonable comparison to the NBA. Parker can choose whatever he wishes, but if he wanted to prove his ambition, he had only one choice.
You make a unilateral comparison between Parker and Donovan, but it doesn't fit. What does Parker have to prove? Likewise with Kovalchuk? It's not "more ambitious" for these guys to be a squad player for a middling team. If these guys leave to play in a league in their home country, that's "less ambitious?"
Your argument has teeth, but lacks sense. It makes sense for a younger player to prove his quality at the top stage. It made sense for Klinsmann to push Dempsey to reach for more in the EPL. But it's not "more ambitious" for Donovan to play for a middling EPL team.
Maybe we can agree to disagree, but just remember that you're wrong.