https://www.metv.com/stories/dan-blocker-said-that-introducing-will-cartwright-to-bonanza-was-a-mistake-heres-why

"I think the network is making a big mistake," said the actor.

From the article:

"However, in Blocker's opinion, there were simply too many Cartwright men."

At that point in the series, it was clear that Pernell Roberts would not be returning to Bonanza. Guy Williams would have been his replacement. Laura Dayton (Kathie Browne) was originally to have married Adam Cartwright (Pernell) and off they would go. However, since Guy was not wanted by Greene, Blocker, and Landon, instead they paired Laura with Will (Guy) and they rode off.

First time I ever heard that Dan Blocker had any sort of "issue" with a new Cartwright coming onto the show.

Yes, I had only heard in the past that Lorne Greene and Michael Landon were the ones who didn't favor the addition of Guy Williams to the cast. I should have left Blocker's name off when I replied, it's hard to believe he objected.

Yes, I had only heard in the past that Lorne Greene and Michael Landon were the ones who didn't favor the addition of Guy Williams to the cast. I should have left Blocker's name off when I replied, it's hard to believe he objected.

It's fine, it's part of the article and it makes sense that all the other main actors who had no plans to leave Bonanza would have had good reasons to not want someone to try to "replace" a Cartwright.