We routinely use germfree mice, rats, and pigs in our research. They are completely devoid of bacterial and fungal cells. Their immune systems develop abnormally and they have some behavioral oddities, but they have no reduction in lifespan or fecundity.
Nutrient capture is really not that large a part of the gut microbiomes role; most of the nutrient and caloric uptake occurs in the small intestine of humans. The small intestine has many bacteria, but maybe 1e-3 fraction of the large intestine. Interestingly, humans have a much larger small intestine:large intestine ratio than close primates. Likely we evolved on such a high quality diet (animal protein) that our need to extract nutrients from plants declined and allowed shortening of the large intestine. When you get high quality feed, you don't need a big bioreactor to ferment plants into calorically valuable products like short-chain fatty acids.
This is not to say that humans aren't better off eating a very plant rich diet. All the research suggests strongly that a diet very high in fiber feeds a healthy guy microbiome which helps resist pathogen invasion and helps create a balanced immune system.
In short: if you could rear germ free humans, I think they'd be just fine, if perhaps immunologically odd.