That was true for every team so I don't understand how you considered this some type of advantage for the Cowboys.
The Cowboys were an expansion team in 1960. Not only did they receive no additional draft capital like recent expansion teams, they did not even get to participate in that year's draft. They slowly built their team for 7 years before their first NFL championship game. During their 20 consecutive winning seasons, two different rival leagues formed that signed active Cowboys. The first one took Calvin Hill, Duane Thomas, Jean Fugett, and Danny White.
"Hostage" is a little too strong. The NFL didn't owe them the opportunity to play professional football. It's not a right.
Besides, there were several other minor leagues if they wanted to play somewhere else. However, they chose the NFL and those were the rules at the time. I'm not going to defend it, although I miss the enjoyment of knowing that player is one of my guys. These days, the guys I loved today may be the guys I hate tomorrow.
Of course, I don't mean real love/hate, only in the sense of rooting for them as a fan.
Landry won 2 Super Bowls in 29 years, that is true. He also won 5 conference championship games and took the team to the playoffs in 19 of those 29 years for a total of 36 playoff games.