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.Why is that "reasonable"?
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?