Right now, the energy in the Democratic Party is all on the progressive side. At Democratic rallies, speakers do not get cheers when they brag about their bipartisan outreach. A scrappy, energetic progressive like de León could rouse the party base in a way that Feinstein could not. (Bernie Sanders showed the way on that score.)
One might argue that, even if Feinstein ran behind a progressive in the top-two primary, she would win in the general because Republicans would coalesce around her. But suppose there are Democrat-on-Democrat races for both governor and senator – as seems very possible. In that case, GOP turnout would plunge. And even if GOP voters did show up to the polls, many would skip the Senate race, as they did in 2016. In this scenario, a progressive Democrat could win the general as well as the primary.