You don't just 'make' L5. You have to prove you deserve it with a whole bunch of showboating. The process encourages self-promoters and fame seekers, not incremental contributors. They fiddle with the knobs every few months and say they're trying to rectify this, but it's never going to change. It's not like you can just do a good job and move from L4 to L5. You essentially have to lead a project. And that means fighting for a project or lead of your own. Which means nobody wants to do the grunt work, because there's no recognition for it. You will be promoted for initiating and leading a new effort, not plugging away at an old one. IMHO this has a deleterious effect on engineering quality and when its PMs doing this, it leads to runaway competing novelty projects.
It's also far easier for an L4 to get competing offers for amount far exceeding what L5s make. That's also how many (most?) people get the L5 level: they negotiate with HRs when they have the leverage (competing offers). Getting to L5 via the promo committee is probably the hardest way to get there.
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.
