I don't agree with you.
First of all, let me say that ALL games have gone downhill. I have several consoles, psx, ps2, gc, pc, gba, and the one I play most? PSX. Also, Megadrive/SNES. They simply don't make them how they used to, which is inevitable. Back then ideas were fresh, and now they're not, they just have better graphics to compensate. It's the same with movies. Most these days just sell on the effects/actors.
My favourite FF is 7. Sure, ff5, 6 were good, but FF7 also had a very good plot, involving characters, deep storyline but it also had amazing graphics which easily gave it the edge. It was the first 3d FF and it was fantastic.
I don't see the big problem with the plot. I think it's better than VI's. Avalanche, the Shinra, Sephiroth. It's a great plot! Combined with the graphics and the near-perfect battle system and huge amount of hidden stuff, it's easily the best Final Fantasy ever made.
Next we had FF8. The graphics were only marginally better than 7, but the characters were quite different, the plot was more adult and it had a whole new land.
I didn't like 9 so much, but that's not to say it 'sucks'. It had beautiful graphics and a cool set of characters, whole new weapon/magic system.
FF10 I don't consider an RPG. It's more an Adventure game, because it's so linear.
I don't see how you can complain about them being similiar...they're FINAL FANTASY games. Are you gonna complain that Gran Turismo 4 is too much like Gran Turismo 3? The FF legacy has several things consistent throughout the series, we're assured of a well written plot/dialogue, lots of cool monsters, a good battlesystem, cool summons, lots and lots of secrets, and good FMVs. They're all set in different worlds with just vague references to the others. Basically, it does what it says on the tin!
If you're a fan of rpg's, then 99% chance they won't 'suck'.