Elections are popularity contests. If the winner of the popularity contest is more conservative than you that indicates the median voter preference is more conservative than you.
I can see why people want to believe that if democrats moved left they would win because in that scenario liberals get to win elections without sacrificing anything. It’s probably not the case though.
It appears in the most recent election black and Hispanic turnout for democrats was lower than expected. These groups are among the most conservative in the democratic coalition. Why would we think moving left would turn them out more?
But that all assumes (a) that the job of politics is to find out what people already want and try to pander to that, rather than to lead and to inspire people and come up with new ideas (b) that those 'wants' are fixed and unchanging (c) that there's a simple mono-valued political spectrum that runs from left to right and encompasses all issues, and (d) that people are uniformly and evenly distributed along that spectrum, rather than clustering at particular points on it, including at the 'extremes'.
Absent those assumptions the idea that just moving towards the politics of the 'winning side' will improve things may be quite incorrect.