Honestly I think they could have gone up to 4 or maybe 5 good seasons if they properly stuck to their story instead of moving around in filler directions to strech more seasons and ultimately sacrificing the plot for longevity.

Season 7 had an amazing story overall (until the supposed reveals in the s8 retconed the shit out of it) and some focus episodes on secondary characters and their backstory that had some of the best writing in the series. And most of season 8 was completely story focused with no criminals of the week.

If they removed the filler from s4-6 entirely and just led to those stories naturally immidiately from the first couple of seasons and paid more attention to tying up the clues into a logical conclusion instead of basically basing their reveals on online theories it could have been one of the best shows out there.

It certainly had the interesting premise, clever dialogue(at least in the early seasons) and an amazing actor in the lead.

The blacklist. I could see the show was jumping the shark ever since season 5, but it took until the awful finale of the 8th to admit to myself the show had died and would never go back up.

As for your other question I usually want to finish the shows I start. For me to fully quit a show it has to seriously piss me off. So far only 3 shows have managed that:

The blacklist

How to get away with murder

Grey's anatomy.