She accomplished exactly what she intended when she left Cheers
No she didn't. She intended to be a big movie star and she failed.
Everyone in Hollywood knows it was one of the worst career decisions in showbiz.

She did her career no favors by leaving Cheers. She came off as somebody with a sense of entitlement. TV was too small for the movie star wannabe. She crashed and burned and her career was never strong again. Failed movies, sitcoms, and guest appearances was how her career turned out.
And she wasn't becoming stale on Cheers. It certainly didn't hurt Ted Danson's career or Kelsey Grammer's career by continuing to play those characters instead of quitting to progress their careers. Sometimes, you have to be honest with yourself about what you do best, and stick with it. Nobody was interested in Shelley Long's career, they were interested in her best work on Cheers.

Far from blending in. She accomplished exactly what she intended when she left Cheers. She distinguished herself as someone other than Diane Chambers
The role was becoming stale, and she wanted to progress in her career. Which never would have happened if she had stayed.
It was a shock to her how much it affected the show and she regretted only that.
I for one was devastated and only occasionally watched Cheers after she left. Kirsty Ally was possibly the ONLY decent replacement. Even so, she was mostly just a shadow of Shelley's performancereprising the same characteristics that she so accurately portrayed as Diane. There was a little truth in Shells performancebut the real Shell is no Diane!

Pretty much exactly what I was saying.

That popular sitcom was nearly 30 yrs,though. And we don't know on what terms she left the show. It doesn't mean her career is over..just different.

I'll agree to that. It's probably a fair statement that once you've stepped off the most popular weekly sitcom on T2000V for several years, you may not be quite as on-the-radar to every single fan in the world, anyway.

"oblivion" is relative I guess. Let's split the difference.

The question at hand (established by the OP's assertion) was whether Long had in fact "blended into oblivion," or whether that was only the OP's impression.

The OP probably doesn't care about being a movie star,and money isn't the big priority for all actors.

such oblivion, in fact, that she continued to act and was hired to be on several dozen film projects and TV shows after that, all the way from when she left Cheers until 2012 (four projects in the current year). I'm just going to bet you'd like to have that kind of oblivion, not to mention that kind of money.