I'm on 191 firmware right now, I wonder what the firmware version would be if I do a factory reset.
191 or 133?
If you perform a factory reset, your firmware will still be .191. The reset will delete all apps and data, but the FW will not change.
If I want to roll back the firmware to 133, how can I do it?
You can't. Only ASUS can and we won't because there's no reason to. Why do you want to go back? Your phone won't work better, it will be worse since we have fixed a lot of bug since then.
"Only asus can" I dont like that kind of policy or thinking, we users arent make any deals with asus. When we buy the product, we dont want that somebody says how we can or not can use the product.
We are owners not service users.
It's for security reasons mostly. There are quite a few logical reasons to have active anti-rollback mechanism and this is what every phone manufacturer does. For example, somebody could steal your phone, downgrade it to an older version that contains security flaws, exploit them and install spyware or access your data. It doesn't sound very realistic but better be safe than sorry. Or to patch unauthorised bootloader unlocking ;)
Answer: "Others do that too", isnt a reason to do it yourself.
I understand why these things are there but, if you buy a phone, then you are the owner of that phone, so basically you need to have full control of it then, nobody else. So if i dont buy a service from Asus what secure my phone to work like they wanted, then i have to be rights to do what i ever wanted to do with it.
Yes we can use custom roms with Zenphone 6 because Asus at last offers us a unlock tool, ofcourse i dont want to do that yet, because rooting isnt what i want for my daily phone (banking apps and stuff), but im glad that Asus offers that tool.. and i hope that they can do that with their tablets too, that we who wanted, can upgrade them to use newer android on them with custom roms.
Power to the users!
The explanation ColorSage gave you is correct but may I ask why you want to downgrade? Sure, there could happen that we introduce a bug in the latest version which wasn't there before but then we will fix it in the next one and if the bug is severe, then we normally release an extra FOTA shortly after to take care of it. This is how it works with every phone manufacturer.