I'm expecting to get to l5 in a large part due to (necessary) grunt work.
It's also unclear what you mean by plugging away. If you make improvements to an existing system, you absolutely can get promoted. But on some scale, those improvements are probably viewable as a "new launch", so that's sort of tautological.
This has improved in recent promo rounds, there seems to be more recognition of this. However the L4 to L5 process has been in the past framed as "L5 owns/leads a project/component." In some smaller projects, or in my 'remote' site, this can often mean having to stick your elbows in, in order to own something, rather than just contributing to something someone else owns.
It's also unclear what you mean by plugging away. If you make improvements to an existing system, you absolutely can get promoted. But on some scale, those improvements are probably viewable as a "new launch", so that's sort of tautological.