You are supposed to be able to do this. Whether T-brd thought otherwise or not.
And as T-brd says in his reply this has been fought over many times. Your opinion is Noted. But that does Not mean it will be instigated now.
The Arsonist Is the early unit to take down city defenses. As stated it is Limited. If the Arsonist is nerfed as you want (and to T-brd's level of acquiescence to your demand) then you will need to build More supporting troops to protect them. So that you Can capture a city. Thereby Increasing the number of units in the game at the time of arsonist usage. Overall Unit glut Is a Major problem. The way the Arsonist is designed Helps tremendously in reducing that Unit Glut.
Do you want to go back the days of the AI building Stacks of Rams on every tile around it's cities? Do you want the flood of units this causes were No game can get past the Ren Era? If that is what you want then by all means nerf the *ell out of them and carry on. But don't ever expect to get to the late game.
But many people just gloss over this and nitpick about being OP. It does get tiresome.