Tom Bladon (463 games) and Behn Wilson (339 games)... them, Therien and Jimmy Watson is the entire list frankly.
Then there is the list of 'Drafted, semi-developed, given up on.':
Murray Baron: 83 games for the Flyers... 903 elsewhere. (traded age 23 (Part of Brindy trade))
Gord Murphy: 261 games for the Flyers... 601 elsewhere. (traded age 24 (Garrey Galley, Wes Waltz... fair enough really.)
Dmitri Yushkevich: 197 games for the Flyers initially...571 elsewhere and 18 more for Flyers when old. (traded age 24 for Zubrus pick.)
Janne Niinimaa: 143 games for the Flyers...598 elsewhere. (traded age 22 for McGillis.)
Kevin McCarthy: 84 games for the Flyers...453 elsewhere. (traded age 21 for Dennis Ververgaert.)
Dennis Seidenberg: 93 games for the Flyers... 522 elsewhere. (traded age 24 for Nedved.)
Joni Pitkanen: 206 games for the Flyers... 329 elsewhere. (traded age 23.)
So of the 11 truly significant NHL D men the Flyers have ever drafted 7 were traded by 24 years old... mainly for forwards.
Even the ones who 'stayed' for significant time with the Flyers:
Wilson was traded age 24.
Bladon was traded age 25.
So that is a grand total of 2/11 good D men they kept past age 25.