It definitely can be, but depending on the application, it might make sense to demarcate 'scale/infra' related senior developer responsibilities from 'business

It definitely can be, but depending on the application, it might make sense to demarcate 'scale/infra' related senior developer responsibilities from 'business logic' type responsibilities. An SRE would care about the former, much less the latter. I'm a SWE SRE, and I don't particularly care to look at the code of individual features being added unless there's potential for a large blast radius, but I do care to make sure if something breaks, there isn't cascading failures. I want things to degrade in a predictable way, or to fail in an isolated fashion. The non SRE SWEs are free to break a single feature, and i'm happy to help them roll it back, but that's not what I'm primarily worried about. Conversely, many of them aren't all that interested in thinking about how slowdowns or failures propagate upstream or downstream to other teams that may or may not understand how things are connected, whereas I do worry about that.