Obviously it's one of the greatest sitcoms of all time. I think though the people in charge made a few blunders along the way.

1-Not working out some kind of a deal that Don Knott's could make movies and still stay on tags that would work for both sides.
2-letting go of Jack Burns (Warren)
3-Not giving more time to great guest supporting actors.such as Ernest T Bass , Lydia Crosswaite, Flora at the diner, fun girls.
4-making Goober smarter
5- bringing on dry characters like emmit, Howard. Even though I like both they weren't loaded with comedic talent.
6- not showing Juanita
7- too much opie
8- dropping Otis on pressure all together and not finding a spot for him
9-bringing on Ken Berry and doing the rfd thing. Had good ratings. But, not funny

Probably some viable reasons things did or didn't happen but just my thoughts.

I'd say the biggest blunder TAGS made aside from not ending at Season 5 was Helen's introduction. Granted she was supposed to be a one-off character, but there's still the fact her first appearance is one where she yells at Andy in such an over the top uncontrolled manner that it makes you wonder if she had an undiagnosed mental illness. The fact that Andy didn't march her right into one of the cells to cool off shows he had a lot more tolerance in the black and white era.

To make things worse, it's Helen's own fault that the kids aren't into history. During her hissy fit she admitted that she was trying to use high pressure to get the kids to where she wanted them to be. The fact that this isn't addressed is downright infuriating, particularly since it would be an easy fix. All they would need to do would be to have her walk into the court house while Andy is telling the story of Paul Revere and have her realize that's how you get the kids interested in history as well as other subjects. However the script decided that Helen didn't need to be punished for her bad behavior, a fact that would repeat itself over and over again.

Helen has a lot of sweet moments, like at the end of the history episode. She's not always grumpy, or crumpy.

I think the character gets portrayed here as meaner than she actually was.

I couldn't stand Helen especially that first episode. Then her sarcasm can really get to you. I like how Flora flirts with Andy at the diner and throws Helen her pie. Any episode where Helen gets jealous was tedious and annoying. I think that lawyer by the pool when Andy has a fake sunburn was over the top. I thought Helen would scratch his eyes out. How did they stay together?

Another blunder was giving Gomer his own show and introducing Goober.

Judy,Judy, Judy

Not letting Ellie be more than a foil/ straight woman and not treating Miss Donahue with actual respect and appreciation for her talent and contributions so Ellie Walker could have wound up being the 2nd Mrs. Taylor instead of Helen Crump whom only Andy (and Mr. Griffith) appeared to like.

One blunder was having a horrible final episode. They probably should have made the wedding the last show. Or they could have married Barney and Thelma Lou for a finale.

That Barney left town to go to Raleigh not keeping in touch with thelma Lou is absurd..
He mentioned at the reunion to her that he meant to write but was too busy.

He didn't have a girlfriend in Raleigh. The episode was hilarious but that part made Barney look like a complete cold moron.

I guess they wanted to crush him so they could build him up again with the Alberta Nelson character that adored him in school.

It was funny but bizarre.

Just watched the reunion episode when Barney comes back to Mayberry and Thelma Lou is coming too. When she introduces her husband Barney looked like he might faint. Great acting from Knotts. He looked like someone had punched him in the stomach. I found his not keeping in touch with Thel and just leaving to go to Raleigh was poor planning. They should have had them have another fight and Barney leaves. BTW did they ever say where Barney was or did he just disappear and only return in a few shows.

Just watched the reunion episode when Barney comes back to Mayberry and Thelma Lou is coming too. When she introduces her husband Barney looked like he might faint. Great acting from Knotts. He looked like someone had punched him in the stomach. I found his not keeping in touch with Thel and just leaving to go to Raleigh was poor planning. They should have had them have another fight and Barney leaves. BTW did they ever say where Barney was or did he just disappear and only return in a few shows.
Got a big opportunity to work in the Raleigh police fingerprint department. It's mentioned when he returns of course but only quickly when he's first missing.

So what was the reason Knotts left TAGS? Was it because he had a 5 year contract and left to do films or did Griffith not cut him in on the success of the show and only paid him as an employee?

I thought Barney leaving was bizarre because to me, he seemed like "Mr. Mayberry" that he would never leave. You can tell when he left the show wasnt the same. I mean Warren was awful. Plus, his introduction seemed like a Barney episode. When Warren arrested the bingo ladies, it's like they crossed Barney out of the script and put Warren.

I've never read anything about any ill will. Just that Dan Knott's wanted an opportunity to make movies. I think his choice to leave the show changed sitcom history.