Taking on a Democratic party icon like Feinstein will be a real challenge for de León. He's going to challenge her from her left, saying she's too moderate for California. There have been successful challenges to Republican senators from grassroots candidates to their right but it's rare. I can't recall a sitting Democratic senator losing a challenge from their left, so it's probably just as rare if not more so.
de León has several things working against him. He's unknown statewide and has no money in a campaign account yet. California is a very expensive state to advertise in even if you have money. Feinstein had $3.6 million as of June 30 and she's well known statewide already. de León could get a boost from progressive groups that know him. The California Nurses Association is a very powerful special interest group and they're likely to endorse de León.
Gavin Newsom is running far to the left in the gubernatorial race and is leading in polls. If it works for Newsom, why not de León? Newsom has advantages. There's no incumbent in his race, let alone a popular incumbent who's built up years of good will with the voters. Newsom is fairly well known statewide. He likely is getting a good share of Democratic and NPP voters who know him and think he'd make a good governor. Those people aren't currently in de León's camp. If he runs to Feinstein's left he's not going to try to get them.
If California's size and Feinstein's goodwill aren't enough of a hurdle California's top two primary creates a bigger one. In top two everyone gets to vote for any candidate they want from any party. Feinstein is popular with people who don't have a party preference and some Republicans. Especially if the alternative is further to her left. While he could conceivably get more votes than her among Democrats, he won't in top two.
The good news about top two is that de León will advance to November whether he finishes first or second. To finish second de León would likely have to beat a Republican candidate. Republicans tend to vote for Republicans, even the candidate is just a name on the ballot. In order to make top two de León actually needs Republicans to vote for Feinstein in the primary. He needs the remaining Republican vote to be spread out enough that no Republican gets more than than he does. His ceiling is probably 15-20% in a primary. de León would really benefit if Republicans don't coalesce behind one candidate, but split them among two or more. Right now the Republicans in the race are names on the ballot. So that benefits de León if it doesn't change.
Then he'd have to beat Feinstein in the general election. While Republicans voting Feinstein in the primary could benefit de León, he needs them to leave the ballot blank in the general election. But if they voted Feinstein in the primary she may have already gotten those voters for the general election. In 2016 two Democrats ran for Senate in the general election and 14% of the Californians who voted for President didn't vote for the Senate. In Pennsylvania that number was 1%. In New York it was 5%. So there is precedent for blank ballots.