I don't think the FF series is getting worse...
IMO, FF2 and 3 were kinda meh...
FF7 is like the best... FF8 was Ok too... didn't play FF9... and then FF10 was also very good...
It's just all opinions... so
to you... if they end the series, you'll end up with a lot of dissapointed people... if they keep going and you don't like it, DON'T BUY THEM...
Besides, for all you know, the next game will be the one that turns the series around and regains the "FF touch" that you think it's lost...