Or maybe when literally 5% of delegates had been awarded, and the race just had a major shake up in several candidates dropping, you should feel free to vote for your preferred choice
Anyway, the false narratives of "Warren should have dropped out so thereby Bernie would have won" are just BS. First, we already did see Warren underperform her polling by a few % indicating some people really did abandon her ship (anecdotally, I sure know people who did that in Virginia). Bernie still generally speaking underperformed even if we add in some of those Warren folks who presumably jumped ship to him. But we also know that less than half of Warren voters have listed Bernie as their second choice in past opinion polling, and this even assumes they'd show up to vote for a second choice.
A majority of Warren's second choice voters have previously stated support of the "3 kids in a trench coat" moderates combined, 2 of whom withdrew and endorsed the third. There's reason to believe that were Warren to drop out, it'd be roughly a wash in terms of how much goes to Bernie and how much goes to Biden. Her coalition (small, at this point), is not all Sanders voters. A large chunk who were have already made that jump. What remains is not entirely clear, but it seems to be much more mixed.
Secondly, it's inherently unfair to apply this narrative only to Warren and not say the same thing about Bloomberg, the majority of whose support goes to Biden (I mean at this point it's probably almost every one who even still shows up to vote for a second choice, since they can't go to Buttigieg or Klobuchar at this point. How many Bloomberg -> Bernie voters are there really lmao)
For example, here's the 538 averages for Massachusetts vs actual results
Biden 18 -> 34 (+16)
Sanders 24 -> 27 (+3)
Warren 21 -> 21 (+0)
Bloomberg 15 -> 12 (-3)
Buttigieg 13 -> 3 (-10)
Klobuchar 7 -> 1 (-6)
Other states aren't quite so extreme as this one I think, but generally speaking, Biden really did consolidate Buttigieg and Klobuchar voters, though a small portion did go to Sanders. Similarly, Bloomberg has generally speaking underperformed his polling, with, it seems, some of that support heading Biden's way too. In some states, Warren also underperformed her polls, but even in those Sanders did not significantly overperform his numbers. It's not clear if she dropped out that it would be much more than a wash in a lot of states.
How much of this is bad turnout assumptions in the polls vs late breaking movement post-SC and endorsements from Buttigieg, Klobuchar, and relevant local figures in individual states I'm not sure. If it's the late movement, I don't really blame the polls since it all kinda happened at the 11th hour. If it's turnout assumptions though that would be more interesting.
EDIT:
seems to mostly be late breaking voters: