It was never one of the "highest" rated shows on television if we're talking Top 5 overall, critically. Its ratings have slid substantially, too. The shift is apparent in its Metacritic season by season ratings:
#1= 77; #2= 81; #3= 78; #4= 79; #5= 75; #6= 63; #7= 81; #8= 71
It was nominated for Best Drama from 2008-2011. The last nomination was for Season 6, but clearly viewers were displeased because it disappeared for its last two years. The finale was one of the most panned and ridiculed finales in all of 'regarded' television history.
The people's votes tell a much more direct story:
http://www.imdb.com/title/tt0773262/eprate?ref_=tt_ql_tv_3
Just eyeball it. It's impossible to miss. There's only 3 episodes in the Top 25 episodes as rated by
show fans that came after season 4, and all are from Season 7.
More telling: 11 of the 12 episodes of the final season are in the worst rated 16 episodes for the show's run. Pair it with the worst of Season 6 and they account for 17/18 of the bottom-rated episodes. Frankly, I can't think of any other show where the Finale is the lowest rated episode, by far, in the entire show's run. IMDb users rated it a 5.5 (relative to the overall rating of 9.0).
The show went to shit Tkotom. Total and complete shit. Everyone else saw that. What's got you blind?