There might be a second path to impact here in addition to the chance that any single vote is pivotal to changing the election. In choosing future candidates and platforms, the parties will take into account the margins of victory. A big win by Harris (or Trump) will be interpreted differently than a very narrow win. The parties may look at this as a measure of where the median voter lies (or some other measure relevant to the elections) and adjust accordingly. Each additional vote will have a small impact on this, but as you point out the scale is large.
So it may not only be a very small chance of a large impact but ann almost certain chance of having a substantial (but very hard to measure) impact.
(This is not my original idea there have been some papers in this in political science or public choice.)
There might be a second path to impact here in addition to the chance that any single vote is pivotal to changing the election. In choosing future candidates and platforms, the parties will take into account the margins of victory. A big win by Harris (or Trump) will be interpreted differently than a very narrow win. The parties may look at this as a measure of where the median voter lies (or some other measure relevant to the elections) and adjust accordingly. Each additional vote will have a small impact on this, but as you point out the scale is large.
So it may not only be a very small chance of a large impact but ann almost certain chance of having a substantial (but very hard to measure) impact.
(This is not my original idea there have been some papers in this in political science or public choice.)