You have some good points about Angle wanting to be a big fish in a small pond, but again I don't blame Angle for wanting to be “the man.” The last 3 years of his career Angle's job has been to put other guys over: Brock, Cena, Eddie, Mysterio, etc. WWE wanted to use arguably the greatest wrestler ever (Angle) to put people over all of the time. So I agree with Angle’s attitude of wanting to leave for an organization that will make him their top wrestler.
As for why TNA uses the NWA belt….it was for prestige. When TNA originally started four years ago it didn’t have any creditability attached to the brand, except for the NWA belt. Now that TNA has established itself and gained creditability it will probably be a matter of time before TNA creates their own belt to replace the NWA belt.
I seriously suggest that you checkout a TNA PPV before you denounce TNA. I have known people in the past who only saw TNA on TV then they see a TNA PPV and they take back their past criticisms about TNA. If you haven’t seen a TNA PPV buy or rent The Best of AJ Styles and or TNA Bound for Glory…watching those DVDs will turn most wrestling fans into a TNA supporter.