For context, the early comment setting up this hypothetical was:

> if the electoral system were more fair.

So no.

Ok, so we'll stipulate to some arbitrarily "fair" electoral system:

What makes it obvious that a leftward shift would grow today's Democratic base to that left _more_ than it might shrink it on the right?

It wouldn’t, long term IMO. The parties would end up roughly with similar overall proportions. Their positions would change. Democrats, with greater electoral power, will focus on internal competition for who is more ideologically pure & thus experience a leftward shift, to the point of probable over-reach (while Republicans, losing a lot of their in-built electoral advantage, will focus more on electability and thus will moderate, making them more palatable for marginal/independent voters), which is the mechanism for restoring the balance of power.