Too much 20/20 hindsight. He picked Sessions in October or November (?) and the Russia thing was gradually fomented by the Dims over several months and wasn't a major issue until Sessions was near his confirmation hearings. And it really blew up after he was confirmed as one of your posts showed.
Trump shouldn't be throwing Sessions under the bus in public. Very poor form and it will make it much harder to find a decent replacement for AG, if necessary, and it'll make it harder to fill other positions that are still open.
I don't have a problem with public put downs, per se. They can be justified and useful. My issue is that Trump is blaming Sessions for not being prescient, and that is only not fair to Sessions, it looks like a character flaw in Trump, to me. And so I agree with your point that this risks turning off qualified people, from serving in the Trump administration. Loyalty is a two way street. Trump doesn't give it, he has no right to expect it.
I climbed down Angus King's throat for accusing sessions of playing "hide the ball" in sessions' June hearing appearance. My calling of of King was justified ONLY because the accusation, by King, had no basis, and King knew it had no basis. He was trying to score points with some DEM constituency, but still bad character on King's part.