I am willing to bet you anything SANRAL will happily let unregistered motorists run up two/three/four months of tolls and then announce that they will prosecute anyone who hasn't paid to the full extent of the law with scare stories and horror tactics.
They will announce that they are preparing 250 000 summonses and even start sending out a few thousand threatening criminal records, etc so that everyone knows someone who knows someone who got summonsed. While everyone is panicking and outraged about this, they will then announce that in a display of goodwill they will offer an amnesty to all "perpetrators" on condition they get an e-tag, register and pay within 30 days.
It is at this point, that, faced with the reality of criminal charges and fines, most will cave in and the SANRAL booths will be swamped with hundreds of thousands of Gautengers queuing to avoid the deadline.
SANRAL are not stupid, they know there is huge public resistance, so they also have probably obtained the best PR/propaganda/mass manipulation advice for herding the sheeple. It is a long term strategy that will be fought over months. If we know what to expect we can come up with strategies to counteract it.