Against Planet-Eating Nanoreplicators

A classic trope of hard sci-fi as well as more serious futurism is using self-replicating nanoassemblers to convert planets of the Solar System to computronium, or some other kind of a Dyson swarm. This is almost the default way to colonize space in any projection of the future that features singularity or ASI, and not uncommon in other settings as well.Except that even if we grant the nano part works exactly as advertised, and even if we ignore the gas giants and only focus on rocky and icy bodies that have a hard surface - which constitute less than 1% of the total planetary mass, by the way - there are at least two very fundamental problems, each one making it basically impossible to use self-replicating nanobots as the primary tool for space colonization: matter and energy. I'll go over each one in turn.(Caveat: this post is specifically about using nanoassemblers to colonize space on planetary scale. There are a lot of arguments for why they can or can not work conceptually, and whether a runaway grey goo scenario is likely within the Earth biosphere. I don't have the expertise in any of the relevant fields to contribute to those discussions. Instead I'm presenting a weaker claim: even if nanoassemblers can be made to work on the nanoscale, they aren't suitable as a planet-shaping tool. I think this claim can be supported just by the basic physics and chemistry.)MatterSuppose you have a perfectly functional self-replicating nanobot design that can convert raw input materials into more of itself, or any other output you desire. You've solved "fat fingers" and "sticky fingers" and ironed out all the other issues. As any other machine - or biological cell - it is composed of more or less fixed list of elements in more or less specific proportions. There may be some wiggle room - a cell can tolerate some variation in ion levels, a machine can substitute some steel parts for bronze or aluminum - but the variation is very much limited. Different chemical elements hav