It is an
educated guess though. Biden generally performs better than Sanders does in polling. There was a SurveyUSA poll that dropped today which had Sanders running ahead of Biden by 2 points in the general election (+9 over Trump for Sanders, +7 for Biden) and it's literally the first high quality pollster I've seen in a long time to show a result like that. The CNN poll that this thread was started on shows the exact opposite.
In the three states that decided the 2016 election, Pennsylvania, Michigan and Wisconsin, Sanders runs
marginally better in Michigan GE polling, whereas
Biden does several points better in Pennsylania and
Wisconsin. Biden has a
very small lead in Arizona polling as well, often considered an alternative to winning Wisconsin (generally considered the least reliable of those three states), that turns into an outright Trump lead when he's pit against Sanders or Warren. Same with
the ever-controversial Florida - Biden wins by 2.7, Sanders ties.
North Carolina, same story, similar overperformance by Biden.
North Carolina, Michigan and Arizona all feature high-profile Senate contests as well. The better the Democratic nominee does in those states, the likelier it is they carry the rest of the ticket.
I'm not saying this as if it's a definitive truth, I'm just pointing out that when people say Biden is more electable, they're not talking completely out of their ass. Yes, these are trial heats being conducted nine months out from the election, they're subject to ebb and flow. Biden could very well turn out to be a dud of a candidate, in fact it is one of the biggest things I fear about him as I do think he's the likeliest to win the primary, but it helps to try and understand the logic behind "Biden is the most electable" rather than just angrily kneejerk reacting to it. You don't even have to agree with it, but it's not based on nothing.
Regarding your last point, no, Biden doesn't support Medicare for All, but
his healthcare plan is quite good. The most important element is it creates a strong public option, which would be the likeliest catalyst for us to move onto a single-payer plan eventually - I support M4A but I think we're still a good 5-10 years away from it being feasible, and Biden's plan would represent a significant step towards it. Biden also has a
decent climate plan that often gets overlooked and made me breathe the biggest sigh of relief regarding his inevitability as a nominee, as I consider climate change to be a literal existential crisis that requires immediate action. Neither plan is as bold as Sanders or Warren's policies, but they are significant and the suggestion that Biden would simply be a return to the status quo is ignorant at best and disingenuous at worst.