Fleurista, I'm curious as to why you posed the question in your OP only to QT - and why you only asked "what would be the worst" impact. Why not ask people from both camps to consider and share what they think all the impacts might be - bad, good, in-between, both - across the whole spectrum from best to worst? Rather than asking the QT side only, and then asking them to focus on thinking and naming the most extreme negative effect(s) called "the worst."

One thing I've observed about many people who identify as trans and are caught up in gender ideology today is a tendency to see things in a negative light, to view the glass as half empty rather than half full, as it were. Or, as I like to put it, to look at the world "through bruise-colored glasses." I fear the way you've framed the question here reinforces the tendency to think only of the negative when imagining the realm of possibilities. (Sorry if what I've said sounds insulting or judgmental or anti-trans and anti-genderist. I don't mean to come off that way. Lots of people of all kinds see the world through bruise-colored glasses. I don't mean to suggest it's a trait that's inherent or limited to trans people and those who are hung up on "gender." Coz it isn't.)

Also, it's not clear in your OP what you mean when you ask what would happen "if the concept of gender did not exist." A topic as recent threads reveal, there's a whole lot of confusion and disagreement about. My hunch is you are not asking what if there were no sexual reproduction, no sex differences, no sex acts. By no "concepts of gender" did you mean no sex stereotypes - and no sex roles beyond the ones related directly to reproduction (like women being the ones to stay home with a baby in its first months/year coz of breastfeeding, the importance of the mother-baby bond in the "fourth trimester" and women's need to recover physically from the rigors of labor and childbirth). I suspect you meant the latter, but one can never be sure.

FWIW, I know you didn't ask GC, but in case you're interested, I don't think a point will ever come when there will be no one on earth who believes in sex stereotypes, sex roles based on biology and sex roles based on sexist sterotypes. Coz there will always be noticeable differences between the two sexes, and humans are programmed to make observations about the world and the beings in it, and to make categories based on similarities and distinctions - and to make generalizations about the categories. That's just the nature of how our brains and psyches work. Also, the vast difference between what human reproduction requires of the two sexes is gonna result in some roles being allocated by sex. For example, I would have loved to have shared the burdens of pregnancy with my husband, and I also would've been happy to have split breastfeeding duties with him. He, in turn, would have loved to have been able to have a baby and to breastfeed. But neither of those things were options. Coz of the immutable realities of biology. So anyways to me, imagining a world where there's no sex stereotyping or sex roles seems pointless and pie-in-the-sky, sort of like those inane "thought experiments" about imagining waking up tomorrow with the body of the opposite sex.

However, I do think it's possible to imagine and create a world in which there are fewer and far less rigid sex stereotypes, where the majority of people take sex stereotypes far less seriously than many today do, where kids aren't raised to feel they must conform to one or the other set of stereotypes, and where no child (or adult) will be shamed, punished or bullied for liking "the wrong" stuff, wearing "the wrong" clothes, behaving "the wrong" way," having "the wrong" affect/mannerisms and personality traits for their sex. I think this would have a lot of positive effects on many people of both sexes. However, I also think it would have a number of impacts that many adolescent and adult males would perceive as very negative and would seriously take issue with. Which gets us to the heart of the issue... Or maybe I should say, it gets us to another mammalian organ, one only the male sex has, LOL.