The reason it is considered unnecessary is that the existing blocks work enough to cover the most of the population, who aren't technically savvy enough and would not bother to take the special effort. This is enough to lower the reach of sources that contain information not approved by the government to where it doesn't pose any danger to the regime. The same concept as "kitchen talks" in the USSR, only on the Internet - you can have VPN in your own "kitchen", as long as it stays there it will be ignored.
So, if Tor browsing becomes easy enough for a common citizen to use, they will disrupt it just enough so that common citizen won't be able to use it, and would stop there.
The reason it’s unnecessary is because current mechanisms are enough to stop Google and FB from making money on Chinese citizens, which makes it unprofitable to spy and manipulate them.
So, if Tor browsing becomes easy enough for a common citizen to use, they will disrupt it just enough so that common citizen won't be able to use it, and would stop there.