Child abuse is just like everything else, low-friction solutions are going to win over high-friction ones. If you have to get an Android phone and sideload a shady app to get E2E encryption, many people won't even bother. In the world where E2E is banned, half of the shady E2E apps would be police sting-ops in the first place, and the other half could be seized and turned against their users with a strategically placed update. Just like with money laundering, you don't even have to prove that a crime occurred, simply getting the list of all IP addresses of people using E2E would go a long way in putting those people behind bars.
Again, I'm not saying that this is necessarily a good thing, but it's undeniable that many children would be saved here.
You could have AI detecting instances of domestic abuse and immediately dispatching a police drone to handle it. Technologically this is almost already viable..
For now, I'm inclined to err on the side of caution and not enable perverts.
A lot of that stuff will be much harder to find out if end-to-end encryption stays legal. Is it still worth it? Very possibly, but it's definitely a tradeoff and we shouldn't pretend otherwise. We're trading off real children being actually hurt against unspecified hypothetical harms in the future. Those harms may be much more significant, or they may not, but we shouldn't pretend like it's all roses.