Why don't you tell this to the producers of serrated knives? I guess they would want to know that they are doing this completely wrong.
Anyway, per the rest of this discussion, definition of 'gatekeeping' is clearly contextual. Paraphrasing the exchange above: "What about safety?", "Sure, but that's not what we usually mean by gatekeeping."
Telling somebody they shouldn't weld without goggles isn't gatekeeping, it's safety advice. Telling somebody they aren't a real welder because they're using a cheap but functional brand of welder is gatekeeping, because there's no good reason to malign a welder for having an unfashionable brand of equipment.
Gatekeeping is only 'true' gatekeeping if there isn't a good reason for keeping that gate; safety advice in particular is generally exempted. Since gatekeeping is only true gatekeeping in contexts where there isn't a good reason for it, claiming that something is gatekeeping without explaining why is basically worthless. Such accusations are assertions without supporting arguments. It often boils down to circular reasoning: You're gatekeeping which is bad, and it's bad because it's gatekeeping.
-edit-
You specifically said a blade would get so dull it would become a saw.
You weren't talking about serrated knives as you said "and the blades of most blunt knives develop a saw-like edge"
Using serration on a knife to saw is not misuse, using a dull blade to saw is misuse and dangerous.
Basic knife safety is not gatekeeping. Sharpen your blade, or at least use the knife alone so you only hurt yourself if you're that insistent on using knives improperly.