Would modifying the human microbiome to digest cellulose be worth it?
You really need to understand how cellulase would effect metabolic systemic homeostasis before anyone can answer that. It's an interesting question. At first glance, one could consider that cellulase would allow disproportionate glucose intake, ultimately making fat a smaller % part of the diet. If unchecked, this could lead to a reverse Randal effect inhibiting fatty acid metabolism thus altering adipokine synthesis leading to a range of negative, if not, species detrimental effects such as reduced capacity for reproduction (as an example between altered biosynthesis and crosstalk between leptin and estrogen). This may have presented a problem for early humans, because in essence, cellulose could have acted as the very first mcdonalds convienience if we were capable of digesting it. I'm sure I'm missing many complications that would have occurred if early humans had this capacity, but it's a nice thought experiment.

And regarding the mathematical induction, forget about it, you can't induce a conclusion on potential alterations within the mammalian digestive system from only the enzyme kinetics of cellulase found in bacteria.