Yes, it is already marketed. However, the caution arises from the fact that if you add fungi that favor certain plants over other plants, and let them reproduce, you might drive ecosystem dynamics away from stability (something like that).
If the concern is about driving ecosystem dynamics away from stability, why does it matter in what way the fungus gives the plants benefits?
This is a place for matchmaking researchers with research ideas in applied sciences that have a sizeable impact on human condition.
Edit: let there be janitors.
What are the so-called adult problems in your field and who, in your opinion, is needed most to solve them?
Add reasons why you are not working on them yourself. (I, for example, am in a PhD program very remote from practical applications and might have a chance to do some research on useful stuff if I survive that long.)