I'm thinking Dark Knight's rework will be more extensive than I initially thought.

[Shurrikhan]

It's easy to claim that something "is not an identity" without providing an actual example or criteria that defines what an identity actually is.

Why should the burden of proof lie with someone not convinced that, as per the norm, a single mechanic does not an identity make? That's not so much a claim as it is a matter "you've yet to show me how that would be sufficient"?
If you've not provided significant context or synergy for the mechanic, then a theme based upon that mechanic will have to be judged purely by the mechanic.

PLD's shield forms a core part of its identity. You can argue that "block isn't an identity, it's a mechanic". That's true, viewed in isolation. But there's more to it than that. You build gauge in Shield Oath by blocking. You have an ability that lets you counterattack after you block. You have moves that adjust your block rate. You have abilities which allow you to take damage in place of an ally, gaining more block procs in the process. Your signature move this expansion lets you stand in front of the group, shield outstretched, to defend your team. The shield is a part of your identity.

Right. So how can we situate Lifesteal to anchor a fulfilling DRK identity? It's not just a matter of groundwork; all groundwork does is mitigate any sense that the job has been arbitrarily or suddenly changed. What are the ends you're imagining that make Lifesteal feel so lucrative to you? If there's more you can give us to explain that, please do. "More ways to steal resources, and more interplay between them" sounds damn good, but where is that going and what will it look like in practice? Until now you've been trying to selling us on a solution to something as complex as, say, Calculus (DRK's massive identity issues) via only something as simple as "Add up the parts" (Lifesteal). While that may technically work, you could certainly give more explanation for how that actually provides all we need and why your method is particularly sound.

Is Lifesteal going to be something always available and used only "as needed", or is there something more to it that we can bank? What will be its actual cost? At present, Power Slash need scarcely exist, thus the only limiting factor for DA-SE is essentially rate of action, and DA-SE provides little of significance outside of extra damage and thus can only really be optimized around raid damage windows. Unless something changes in the actual gameplay, priorities, and possible optimizations I don't think Lifesteal will offer that much to identity. What more can we do with that mechanic to really augment gameplay?