I really enjoyed The Big Bang Theory but it started going downhill around when Penny got her haircut short. I'm not blaming that for the decline in quality but as the characters got older and gew up, the show seemed to get dumber and it went from a credible real-life comedy into some cheap sitcom with lots of simple humour where the characters really began to irritate and ideas were thin.
I think season 8 onward marked the decline, plus the actors were getting exceptionally greedy in their wage demands - something like $600,000 to $1m an episode with even fringe characters earning six-figures in the latter seasons. Some of the later episodes were horrendous and they introduced so many pointless new characters and gave previous fringe ones larger roles which didn't expand or freshen up the show.
Credit to Jim Parsons for leaving and missing out on potentially $40m but he knew the show had gone as far as it could. Two more seasons and its legacy would have been completely ruined.