Most likely it should be possible to write an extension to change this behavior.
Regarding the challenges of implementing that, one is changing the routing (probably not too hard), and the other one is making sure no tag slug clashes with another route of Flarum. It's particularly complex when you see that extensions might register their own routes that could be identical to tag slugs.
The latter is why we will probably not offer this feature as part of the core extension. Using a prefix requires less code and less checks, and it will work fine with many other extensions.
Feel free to publish your work as open-source, others might benefit from it !