If you’re locking yourself in to voting for one of the two major party candidates to even have a chance at moving the needle on policy, then I would argue you’re not multiplying a small probability by a large benefit because the major parties actually agree on almost everything and only clash on marginal execution issues (the occasional brexit type referendum notwithstanding).
For all the anti-war protests in the Bush years, Obama kept them rolling at pace for almost a decade; for all the tea party protests in the Obama years, republicans had no fiscal restraint when given the purse strings. These were just the noisiest issues on each side (and may be interesting to explore deeper in relation to the value of demonstration) but the list of policy more-or-less agreement between them is nearly endless. So the benefit in normal (non-brexit) elections is marginal at best.
Thinking about this more in the shower the following occurred to me:
3rd party voting meets both conditions. As a vote, the potential gains are much larger both on nominal alignment and execution (as non incumbent parties are (I imagine) more likely to take action to disrupt the status quo if elected).
But 3rd party voting is also a form of demonstration, enticing mainstream parties to align more closely with your highest value positions (at least nominally) to win proven (and numerable) voters. E.g., if you’re a Democrat with special consideration for environmental issues, a vote for the green party probably has a bigger influence on democratic environmental policy than a vote for a moderate democrat (with no included signal that it’s environmental justice motivating your vote) paired with attending an earthday rally. Maybe?