I don't see "DevOps" as a way to replace some roles - but as a way to make everyone work better together. Instead of living each in their own bubble (and in my - pretty limited I admit - experience it always) everyone has to know, at least a little, what someone else does. It really helps everyone at the end of the day.
And the developer can keep coding without me screaming at him because he placed the database connection string in a configuration file that sits inside a .jar that sits inside a .war and so on.
I'm just a guilty as anyone else in try to write code to fix a problem that's better handled by existing infrastructure and servers. I worked on a project to deliver invoices to customers in downloadable form. In the end pretty much very thing was thrown out and I just need to write the authentication part, because the sys admins pointed out that the existing StringRay boxes could handle everything else (http://www.riverbed.com/products-solutions/products/applicat...).
It's not that it doesn't make sense to have dedicated developers and systems administrators, as the developer you just need to know enough to be able to talk to and understand the admins position and thoughts.
"devops" is having developers sitting with, understanding, architecting, and in the end programming solutions to what were traditionally ops/sysadmin problems. and operators sitting with, understanding and participating in service architecture, teaching about livesite realities, coding where possible, and appropriately buffering devs from noise on the livesite.
the unfortunate thing is that many companies swing the pendulum too far one way or the other. neither all devs nor traditional dev + IT/ops orgs are the best way to build a great product and run a world class service.
However, it's obviously better to separate matters and offload management/administration tasks to a separate team/person. Thus, a good developer in a good company (which has a separate sysadmin roles) indeed can't really replace good sysadmin because the latter has niche practical knowledge on handling various situations (especially emergencies) quickly.
Nonetheless, one can be both a good developer and a good sysadmin at the same time.
I suspect in most mid and large sized companies that the top 10% of developers are creating more value than the next 50%, and that below that, the value could easily be negative.