It was, but it wasn't.
The things that made SEED popular, the friendship stuff with Athrun (who was Char, but not really, I mean Athrun and his relation to Kira took influence from 0079 Char -> Zeta Char, but it still felt pretty unique and when Athrun joined TEAM.GOOD.GUYS with the reach out hand grab from Kira on the space shuttle launch rail it had a lot of umph), the melodrama like Fllay sex antics, Lacus and Bart Ship Captains (Bart obviously is Rhomba, but having him come back on TEAM.GOOD.GUYS was super satisfying) and the incredible music and great battles, great mech designs & upgrades all had 0079 influences but SEED's still its own thing and it worked.
Actually the more I think about it, I think one of the reasons SEED just felt so satisfying by the endgame battles is that every likeable character ended up joining TEAM.GOOD.GUYS and saying "fuck both sides, we're gonna wreck shit in the name of FREEDOM & JUSTICE" and then they jump out with super crazy meteor dendrobiums and giant laser swords and it kind of hits all the right notes to make everything really satisfying in the end based on all that the show built up. I mean even Deacca joined up on team good guys and got the hookup going on because he was a character the audience liked. The last 5 eps of SEED are all about satisfying the audience and that leaves a great impression of a series to go out on a note like that.
I think these days a lot of entertainment focuses too much on shitty things happening all the time and then barely overcoming and winning in the end. Having more things go right like the last 1/3rd of SEED (even Fllay who the audience hates gets to be blown away! +1 and then used as an awesome transformation scene with amazing music for Kira to find his way and become FINAL UPGRADE MODE KIRA +2) is a nice thing I'd like to see again in Gundam.