He says that John Kerry in his last visit told M. b. Salman that M. b. Nayif was a “red line.”
The phrasing could be an exaggeration by Mujtahid or his source. What really happened (if true) would have been something like the following: John Kerry in his last visit told the Saudi king that the US did not support moves that risk division among the Saudi family.
If the US is really warning the Saudi king against removing M. b. Nayif, then this is a good thing, because it will force the Saudi king to follow another path in order to make his son succeed him.
The Saudi king has already made big reforms in his government, and it is obvious that the issue of succession is the primary motive behind those reforms. The US is doing a good thing by not removing that motive. As long as the motive remains, the Saudi king will probably continue his reforms.
Hopefully the Saudi king will eventually appoint his son as a prime minister. If that happens, it will be a good step towards reform. It is hard for reformists in Saudi Arabia to demand an elected king, but demanding an elected prime minister is not as hard.