It's not that simple. /r/politics isn't fully representative of all their backers. While they might gain supporters, they might lose more votes.

In Morning Consult, Reuters-Ipsos and Washington Post-ABC News polls, more Sanders supporters name Biden as their second choice than Warren — and a higher percentage of Warren voters pick Kamala Harris as their No. 2 than Sanders, according to recent surveys.

https://www.politico.com/story/2019/07/12/sanders-warren-voters-2020-1408548

If Harris drops out, her supporters might flock to Warren. But if Bernie drops out, his supporters might flock to Biden. It really depends on what happens to Harris and Biden after the results of the New Hampshire primary. It is still too early to strategize distribution of electors. You need 15% in each state.