Actually, in some ways it's easier for those of us who work in the infrastructure groups to demonstrate monetary value. Google has a very large number of machines, and an even larger number of hard drives. Hard drives have fundamentally not changed seek times in over a decade, even as capacity has doubled (up until relatively recently) every 18 months. So more often than not, for many work loads (not just at Google, but across the entire industry) are seek constrained, not capacity constrained. So when Google migrated from ext2 to ext4, it's actually pretty easy to calculate how much money was saved by utilizing disks more efficiently, and the team which spearheaded this _was_ in fact recognized by the company and by the founders.
As far as the need to continue to derive new value to the company, of course! This is true everywhere; the phrase "what have you done for me lately" is one that is not unique to any one company, and would _you_ respect someone who did one great thing many years ago, and then proceeded to rest on his or her laurels?
Of course there are many different ways of adding new value. If you can demonstrate how a new cache server is faster or more scalable, and thus can drive value to the company, that's certainly one way. Or maybe there is new technology, such as faster networking technology or faster flash storage, which means that assumptions made five years ago are no longer true, and that can be a justification to rewrite some part of the infrastructure. But Google is a data-drive company, which means you need to be able to justify why the rewrite is necessary.