The merger was in 2003. I didn't care for 8 and 9 was just okay either, but keep in mind that Squaresoft was more than just those games. Pre-Enix merger, you've got Secret of Mana, Final Fantasy IV, V, VI, VII, Final Fantasy Tactics, Vagrant Story, Chrono Trigger, Chrono Cross, Super Mario RPG, Secret of Evermore, Saga Frontier, Bushido Blade, Brave Fencer Musashi, Front Mission 3, Parasite Eve, Xenogears ... I'm not a fan of a couple of these, but they were pumping out greatness at a steady flow.
Final Fantasy X and the original Kingdom Hearts were among the last few games they put out before the merger, and while I would gladly list those as great games with the above, you can see within them the start of where Square has gotten itself today. I don't think Enix worsened Square, it just so happened that Square's descent began around the time of the merger. It makes a convenient marking off point.