Why on earth would you bring up Clinton to support your argument? She did exactly what you described and somehow managed to lose what should've been an extremely easy election. Biden managed to win by a very narrow margin in another extremely easy matchup. Not included in your data set are any candidates who ran more to the left, such as Obama (though he governed far to the right of how he ran).
There's so many more disengaged voters than swing voters that it doesn't matter if swing voters are worth more. Besides, swing voters don't just vote according to a rational policy calculus of centrism. A lot of it is vibes or superficial nonsense.
The dems are not going to magically move left, against their donors interests and the interests they've repeatedly demonstrated they hold, just because they win. Especially if that win comes through unconditional support from the left. They are not your friends, and they don't share your interests. They're careerists pursuing their own advancement.