Thanks for the ideas. If I go with the aluminum I'll keep you updated. I just figured the grate would not rust in my Weber's and always looking to try something that might work better. The stock Weber grates are cheap but rust and same with the aluminized steel ones from Old Smokey.
If a rusty charcoal grate a practical problem, know that they will last for many years, even deformed. (And you can extend their life by flipping them over every so often, or step on them while hot with a 2x4 to help flatten.) And after all that, they're still cheap to replace.
At the risk of assuming here, I suspect it's a cosmetic objection. But aluminum will not look quite right after its first usage. It'll probably hold up to the heat if it is thick enough, but then costs rise. If the supporting grate strap has any rust on it, the iron will react with the aluminum and begin corrosion there. Humidity will eventually attack it and begin pitting. That's aluminum's version of "rust" (which is technically only an iron / steel issue).
My position here is not that it can't be a good charcoal grate; I have no clue. But I'd wager it's not a "better" grate overall. Different? Sure! Go for it and let us know!
And again, that Killa Grilla 304 SS charcoal grate is the only solution that ticks all the boxes except initial cost.