Not valid. They may not have really pursued anything else and/or they may have been too closely associated with Star Trek to be cast in any other part. Hollyweird does that with actors who take a part and make it their own - that's what all of the actors on Star Trek did and they will forever be known for those roles.
It took Shatner YEARS after Star Trek went off the air to find the role of Hooper(?) (whatever his police show was called) on TV because of his role as Kirk.
In fact, Nimoy did little or nothing in front of the camera after the show ended. He became a writer/director and only returned to the screen for the Star Trek movies.
Walter Koenig had a recurring star-turn in “Babylon 5” for several years, and gained a huge following among younger sf fans.
Everything I’ve seen indicates the second-tier actors DID pursue careers, just unsuccessfully. Shatner WAS type-cast, and it did take him years to overcome it. He did so by taking any and every role he could.
Over a 40 year span, I wouldn’t consider a recurring role in a Star Trek spin-off as being a success. Nimoy, as you pointed out, didn’t try much acting...but he has had success in the areas where he applied himself. Except poetry...
Star Trek had 40ish minutes to tell a story. It was canceled in the second season, brought back to life and struggled on for a third before dying from network neglect. There was no way it was going to flesh out into an ensemble show. It needed focus, and Shatner’s ego - which was big - forced that. I believe it made ST a better show.
Shatner has tried to make up with the other cast members for years. After 40 years, the second tier actors need to get over themselves.
Actually, Nimoy starred in the TV show Mission Impossible immediately after Star Trek. He played "Paris", filling the "master of disguise" slot vacated by Martin Landau after he left the show.