The big downsides are:

1. Yet another Internet tax on the poor
2. Everyone gets worse internet
3. Most people are woefully unprepared to shop for a VPN from a reputable provider, meaning most people are going to end up subjected to even more surveillance and potentially attacks on their devices.
4. Most people also are not prepared to manage when to and not-to use it, so will get slower internet all of the time.
5. This will create lists of VPN users at both the ISPs and the VPN companies that will be usable by later christo-fascist governments.
6. This will ultimately get laws passed against consumer-facing VPN services, making it harder for people to protect themselves from other attacks.
7. More honey-pot VPN services will start up to support that list-gathering for future christo-fascist governments looking to build dossiers on as many people as possible.

I could go on and on. And nothing about the above has anything to do with porn.