I'm not certain Steyer's entry will scare off many opponents. The statewide officeholders are well known and he is unknown. He could overcome that but they won't know unless they try. It's possible that any of them could decide they prefer the governor's mansion or wait until Feinstein retires in 2018. Of course there's no guarantee another rich Democrat won't get into either race and losing in 2016 doesn't preclude a 2018 run.
His entry might discourage congressmen and mayors, but I'm not sure it will. There are a lot of people who want the senate and there's no point in not trying. In 2010 I asked New Mexico congressman Steve Pearce why he was giving up a safe House seat to run in a bloody primary against Heather Wilson and then run as a Republican in what was going to be a big Democratic year. He replied, "Because it's the Senate." I believe there's a saying that "every congressman wants to be senator." And a congressman can always enter now and then run for his congressional seat if he or she doesn't gain traction.
And congressmen are considering. There's John Garamendi, Loretta Sanchez, and even former rep Ellen Tauscher. I'd be surprised if at least one or two don't get in, especially Sanchez. Some people have said that a congressman represents too narrow a constituency or that they won't be able to raise the money some of the other candidates. First, it'll be a fractured field. Any congressman wouldn't be running one-on-one against another Democrat. She'd only need 15-20% of the vote to make top two. There's also likely to be at least one Hispanic in the field. They have a built in advantage with Hispanics, who are a key Democratic constituency. And Spanish language media is far less expensive than English language.
The word for a while is that Kamala Harris and Gavin Newsom is that they both want to be governor. There's never been anything saying either wanted to be senator. Newsom decided against a Senate run. Harris is non-committal. The problem with each wanting to be governor is that the time for both of them is 2018. Who wants to wait until 2026? How do you stay in the public eye if you do? I'd think they'd both run for governor in 2018. Of course that'd pit them against each other.
I still maintain that Treasurer John Chiang would be the man to beat. No one is focused on him, however.