As to Walsh’s argument, leaving aside the irony of the “binary choice” argument, how many potential Biden voters are pulling the lever for a small-government pro-lifer? Even on issues where Amash is outside of Republican orthodoxy, on foreign-policy and national-security issues, he’s far from Biden’s position. But that even assumes that people are paying attention to Amash’s actual message. Walsh seems to assume not, and Amash’s withdrawal is, even from reading his statement, a concession that his message would be buried, and the only thing people care about is “Justin Amash, he’s not Trump or Biden.”
The last major vote-getter before Gary Johnson who ended up garnering a large-scale vote mainly as a protest was John Anderson, a disaffected liberal Republican who ran in 1980. Anderson’s support was probably disproportionately drawn from people who might have voted, not for his own party’s challenger (Ronald Reagan), but for the incumbent. Either way, though, Reagan beat Jimmy Carter because of how voters reacted to Reagan and Carter, not because Anderson gave a home to people who’d had enough of Carter but weren’t ready for Reagan. If anything, candidates of this nature have a larger impact downticket, by giving people an excuse to show up and vote even when they don’t like the people at the top of the two major tickets.
Amash is an interesting, sometimes infuriating, idiosyncratic politician. But he wasn’t willing to run just as a placeholder. There will be placeholders anyway, and they will not decide the election.