[NiLuJe]

As @DNSB said, basically, KOReader & Plato are document reader apps that implement a subset and/or a superset of what Nickel also happens to be doing, while KFMon is a launcher that piggybacks on Nickel to launch those.

Unlike on Kindle, where we keep the underlying system alive, Nickel is killed in the process when such a non-trivial app is launched. But we do rely to some extent on the Kobo software stack for simplicity's sake.

TL;DR: While it is doable (and has been done in the past, c.f., okreader), you'd effectively gain nothing except shaving a few seconds off a cold boot by switching to a custom barebones system. As no-one in their right mind should ever turn off an eReader that's seeing active use, that's pretty much a moot point for 99.9% of the population.