They really should have ended it after the season 3 finale which would have been the perfect way to go it.
if they should've ended it anywhere it should've been episode 13 of season 4 chuck vs. the push mix. tied up every lose end and gave a satisfying conclusion.
that aside I am not mad they kept going. the last season and a half weren't "garbage" as you say. they were better than what most other shows put out. they could've kept going on for multiple more seasons and I would've loved every minute of it