but they want it different than RS3's hardcore mode...

Ehh I don't think they said that because RS3's hardcore mode doesn't work like that

In RS3 when you die you can't login to any world (but you can still play OSRS and use the lobby), but you can buy an item that when consumed will turn you into a regular Ironman on death (exactly like what we'll get, but already automatically applied in our case). They also have extra lives which I think is what they're trying to "avoid," according to Kieren.

I think their reason for not doing what everyone's suggested is that they just don't want it to be "too much" (note on the quotes) work to be honest, which is a really bad decision in any case

I believe they don't want to give people a reason to cancel subscription on their HCIM account should they die

That is also possible, however I do wish they would allow an option for your account to be reset since it'll bring that "hardcore feel" from different games as well. Although as they've said, they can't, and it's hard to make them change their decisions in cases like these especially since this is just their "filler content."