By definition, the nodes in the network are the participants.
Only people running nodes can participate in the network. What those people choose to accept matter. Self validation of your coins in fork that has 2 participants is pretty useless.
The buck stops with the people participating by running nodes. If tomorrow everyone else but you moves on to a different implementation incompatible with yours then your coins are useless.
Implementations make no difference unless they propose a different consensus spec.
Running nodes only enforce consensus at the moment they validate their transactions otherwise they are irrelevant. If that was not true you'd be able to Sybil attack bitcoin with node count on EC2
It makes a difference in meat space. If an implementation makes it illegal for people to run nodes then people are less likely to run nodes. It doesn’t matter that consensus rules didn’t change.
If all individuals that run nodes suddenly stopped tomorrow because they credibly feared being charged with possession and/or distributing of CSAM, and instead used a single node from a centralized institution do you think that’s good or bad for Bitcoin?
What makes Bitcoin unique is that we can be sovereign and don’t need to rely on a third party for verification or broadcasting of txns. If that is no longer the case then we are just back to relying in third parties. How is this not obvious?
