She was always going to be next in line in this predicament. VP supersedes; if this was a situation like in 2020 where the Dems didn't control the presidency and were all hopeful candidates looking to be the nominee, and Biden had to step down after accepting the nomination then yeah, there would've been another convention.
Imagine if this was 2020 and Trump (sitting President at the time) was in Biden's shoes and had to step down, Pence would've automatically been taking his place in the campaign in the same vein as Kamala took over for Biden since they're both Vice Presidents and supersedes any congressman, congresswoman, or governor.
Anybody thinking otherwise is a delusional Newsom groupie; none of the other candidates would've had enough time to round up the necessary funds unless supported on their dime. Nearly 80% of Democrats in Washington supported her to be the one to take Biden's place in the campaign.