It seems questionable if inoculation will actually work. The soil will already be occupied with microorganisms. Then you are introducing one that you think is better. One issue may be that mycorrhiza could be very species-specific. And the product you have doesn't work with your bonsai species. Another is that you add your innoculant, but they never get a foothold because they are out-competed by what is already there.
Same kinda with Bacillus thuringiensis. The third is that culturing the microorganism so it can be harvested as a product may change the beneficial qualities. Your wild-type mycorrhiza is a strain that has always lived in say pine tree roots.
But now you get a product that was cultured for many generations. It was indeed first isolated from pine roots and determined to be superior to the average mycorrhiza. But culturing it made it grow on a growth medium for many many generations. It would be a completely different selection pressure than living in actual nature in the soil. I don't know if they can culture mycorrhiza, but for sure with Bacillus this is an issue.
These things are now added to the higher priced potting soils, which kinda justifies the pricy nature of the more exclusive product. But in a way, it is just expensive potting soil for people willing to spend more money so they feel better because it is for a special plant. If there are actual studies that plants perform way better with added microbiome, or more basic that indeed the microbiome that you innoculate with actually becomes dominant.
It would make most sense to inncoculate when you have currently a sterile environment. Just saying 'nature can do it by itself' is true, but that doesn't mean we can't improve. In the same way, just because something is for sale and expensive, that doesn't mean it actually works.
It reminds me of the gut microbiome (and I guess skin microbiome also), so my guess is similar principles operate for soil microbiomes.