1. The strongest arguments in those areas are by analogy. Analogising is much easier with fellow humans or even animals close to us than worms. “If you were X, you probably wouldn’t want to be discriminated against” or “they probably suffer like me, we should avoid that”. This starts to break down around the level of shrimp and then is completely broken by the time you get microscopic.
2. Nematodes being morally significant is far more disruptive and absurd than any of these ideas. Accepting that the Old Testament can be ignored on yet another moral issue is pretty easy. Accepting that human welfare is a rounding error compared to microscopic worms is society-upending.

“If in a few decades we do the thing that you don’t think is possible, will you admit that it was possible”

Sure, would also admit I was wrong if researchers find an answer to “why is there something rather than nothing”, which I also believe is unanswerable.

Even if you get a confident answer to these questions, which I’m confident you won’t, the outcome would inevitably be so absurd (nematodes immediately becoming moral priority over everything else) that society would have to discard them anyway or else collapse 

How are you going to decide whether a nematode experiences pain or pleasure, and if they do what is painful or pleasurable to a nematode?

I really don’t think so. I cannot conceive of research that would clarify whether a nematode life is net positive or negative 

If you find some unused ground, plant some seeds, grow flowers, cut the flowers, and make a beautiful bouquet, what did you “take from others”?

Lots of wealth is not “zero-sum” like you describe, but creates new value/beauty/usefulness from thin air and sunlight.

Wealth creation is not inherently bad. In fact if it improves average quality of life around the world (which has happened overwhelmingly since the Industrial revolution) then it’s overwhelmingly good.

Saying crazy but philosophically valid things is fine as long as it’s useful. Many of our current morals would have looked crazy 300 years ago, so I’m glad people spoke up.

Nematode welfare is not productive conversation. The conclusions are clearly not tenable, the uncertainties too broad, the key questions (is a nematode life net good or bad) unanswerable. What is the purpose?

The distinction is that discussions about civil rights, women’s suffrage, gay marriage and trans rights are productive. You can say concrete things, present evidence, and make suggestions for concrete policies that would improve lives.

Nematode welfare is a dead end. We can’t even decide whether nematodes lives are worth living, among various other wide uncertainties, and there’s no way forward to getting any clarity so it’s just reputational damage without any benefit.

More important than isolated concrete examples are the general trends, the thousand little nudges. Can't measure this unfortunately.

Here's an example of a tweet with 5k likes mocking someone suggesting that the welfare of barnacles that are being scraped off sea turtles is worth considering. Likely some of these 5k people were nudged away from caring about animal welfare concerns when they saw this.

I also think of this recent meme on twitter: It's a misunderstanding of a research study that was done on conservative vs. progressive moral circles. Many disdainful memes being thrown around about "the libs" caring about rocks and trees more than about their own family members. A post on the moral importance of nematodes would nudge them further down that belief track I think.