What it comes down to is that comets must have grown, otherwise they wouldn't exist. The scientific consensus is also that their active life is very finite - ie. That within a few million years of having reached the sun's neighbourhood it would either collide with a planet or the sun, or lose all its volatiles, or be ejected. This presumes that the movements, and thus the destiny of individual comets are completely determined by essentially random factors. The trivially nonrandom influences, such as outgassing, spin rate and YORP accelerations can in no way mitigate against this to have comets with features that allow for its continued survival. Essentially it is an "antidarwinist" philosophy. All comets are equally destined to die, regardless of their underlying differences and trivial nonrandomness.
If we are looking at the comet for a life cycle, we are looking at a tiny slow motion sub segment. If we didn't know what a seed pod for a Brasil nut tree was, it looks lifeless and incapable of growing. Imagine if we could only see different pods at different times after being dropped from a tree, and never be able to see the tree, or even inside the pod. There is hints that the seed pod could break up into smaller seed pods. Clearly seeing where the seed may grow is crucial to working out whether we are looking at a rock, a living thing, a dead part of a living thing, or something which living things could be inside. Failing seeing growth, we could look at the chemistry of its surface, try to see what is inside. However, the two basics of life - reproduction and metabolism are the most crucial.
Noting that brasil nut seed pods are similar to each other, and different to more obviously lifeless rocks around it is an important technique. Also the porosity of the exterior substance, the evidence that it has a shell and an interior that may have different properties.
This may indicate that if comets are living, it is the internal payload that is the most crucial, and that the only stage that we are seeing is the dissemination of panspermia comet seeds, and that we are seeing it in extreme slow motion. Maybe a molecular cloud passes by every few million years, and the cometary seeds that are lucky enough to find themselves in that cloud get a chance to grow, hence the importance of spreading the comet seeds widely, so that some get a chance to grow to keep the life cycle going.
This is highly speculative, of course, and it has nothing to do with the evidence that can falsify or fit with reproduction and metabolism.