ok so I'm not very keen on metallurgy is the pros of a as grate just longevity?
in theory if you had a plated grate in your kettle covered and cleaned regularly would it still develop rust way sooner then the SS option?
Yes..longevity is the ss value...and as your post mentions..time to care for non ss grates is not needed...how you value your time and energy is up to you.
Think about hiw many time you grill and need to clean your grates ..multilly that times the longevity of weber oem ss grates and its a personal decision on whether the cost diff is of value to you.
If you care to dive deeper...
If you like grate marks on your food...steel allegedly conducts heat better so nickel plated steel grates are better for grate marks but......most whom manage to keep their steel grates lasting past std rust stage of 2-3 years ..will season their grates with black coating.that black coating is really layers of old food residue and carbon..its up to you if you are comfortable with cooking your good on "seasoned steel grates".
This seasoning is slowing down the rust cycle..not 100% preventing it..
Diving deeper into the seasoned steel grates...the seasoning buildup..removes the value of steel grates offering better grill marks bc the seasoning diminishes the heat conductivity of the steel grates
The otherside of the steel grates is that if you thoroughly clean then after each use...ie..sponge and soap water back to squeky clean condition...they will probably not last a year given that this tyoe of cleaning cycle will wear accelerate the wearing of nickel... a OP on webers website complained of such and posted that their steel grates started rusting within several months
Nickel plating is allegedly harmful...the nickel is wearing off during cook usage...probably some in the food and during cleaning and rusting cycles.
Btw...the weber oem grates are non magnetic .i tested at various test spots on the cook bars..handles and outer ring of the cook grates...