Yes, but what's the ultimate stepping stone to being the president? Put simply, being the party's candidate for presidential election. If someone does not achieve that, they are effectively guaranteed to not be president.You can see the rest of my edits for that. If you discount the founding fathers, you're down to 4. If you cut it down to the modern political scene, you're down to 2. If you weigh that against the number of VPs who have tried to become president and lost the election, results are in favor of VP being a shitty path to the presidency.
Which job has done better? It can be assumed that the vast majority of presidents after the founding fathers will have been a senator, state governor, or perhaps leading general - these are the territory of minimal qualifications. I'm not going to count them so I'm just going to throw out a not-too-implausible figure, but let's say there have been ~1000 senators postwar. How many of them made it to the presidency? What's the hit rate for Secretaries of State, Secretaries of Defence? You can say VPs don't amount to much, but they're pretty much better than anything else.
Politics needs a pragmatic long game, and the job of progressives is not to sulk in a corner because a centreist won their party's nod. It's about building up a future candidate to represent them: public attention, grassroots campaigning, people to rally round. It's not just building up potential candidates, but that that anyone in a position of power can also use their position in turn to bolster the wider movement, influence the party and its machinery.
Do you know how many Democratic Senators are in the Congressional Progressive Caucus? One, and he's already 78 years old. Progressives have a void of people in more senior positions to build upon, and you need to think about preparations for 2024. If you're planning on a new candidate arising before then, they're going to have to be Obama-level charming and inspiring (which is to say one-in-a-thousand amazing, so don't count your chances), or they'll have just three years to build a platform and they'll face a welter of criticism for inexperience. Getting a progressive VP hands you a big shot of a potential 2024 candidate on a platter, should Biden not seek a second term. And more influence and ability to work on the party for 2028 and beyond.