It should be clear that this is a purely symbolic gesture. The bill will die in two weeks at the end of the 116th Congress.
Amash is retiring at the end of his term; he would have had to run as a Libertarian against a Republican whose name is on half the grocery stores in the state (Meijer).
If you're insinuating that he thinks he couldn't have won, I think you're very wrong (both that he couldn't win, and even moreso that he would think he couldn't win). His region has been highly supportive of him for a long time and he's been very up front about his ideologies. Also, the campaign finance numbers from the beginning of 2020 before he decided not to run again looked far better for him even as an independent than it did for any of his Republican or Democratic competitors (in particular, he raised over 2x as much as Meijer in 4th quarter 2019 and also more than all his opponents combined, again as an independent at the time) [1].
The real reason he isn't running is because he spent 10 years in congress trying to make change and has decided it isn't the most effective thing he can do anymore. Congress is too focused on partisan politics and isn't interested at all in fixing its broken system (most of them seem to prefer the broken system), which leaves little room for anyone of any party to make real change unless they're the party leader (or in cahoots with them). I suspect we'll see him running for other positions in the future and wouldn't be surprised at all if he wins whatever race he decides to join next.
I agree with you, he mainly dropped from the race because it was no longer an effective avenue to advance his ideas.
I do have to contest the first part of your statement - it's wishful thinking to see him having won this race. You're correct that he enjoys a broad base of bipartisan support, but the combination of turning on Trump, leaving the Republican party, and having a well-recognized opponent imperiled his chances in that district. He would have had to snag approximately 25% of the Democratic vote and 45% of the Republican vote to achieve that feat.
Fair enough. I definitely think he had a real shot, perhaps not the favorite but definitely a strong contender. He's been a name in his region for so long that I don't think recognition would be a huge problem (even before he went to congress he was in the Michigan house) and the fundraising numbers I shared just don't make sense if he truly had no chance. But impossible to say what would have happened, so I guess we'll have to wait and see when he does decide to run for something else.
Amash is retiring at the end of his term; he would have had to run as a Libertarian against a Republican whose name is on half the grocery stores in the state (Meijer).