This is simple:
1. Most people will not actually "go dark" because the consequence of going dark is you lose everything if you lose your password. That severe consequence for a relatively common human error is not a good fit for most people's personal records and photographic life memories.
The right fit for most people is
(a) unbreakable security on their physical devices so they don't have to worry about getting hacked if they lose them, plus
(b) cloud backup that can be recovered by a trusted custodian, so they don't have to worry if they lose their password.
And that is exactly what Apple is providing. Law enforcement will still be able to go after their backups.
2. As for the case where someone really does want to "go dark", weakening physical device security isn't going to stop them. They will simply use alternative encryption software. Law enforcement still can't get it. So why make everyone more vulnerable to the hacking of stolen devices?
Case in point: law enforcement did get access to the terrorist's last iCloud backup. And if he turned off backups with the conscious intention of going dark, then even if Apple made that impossible on the iPhone, he would have simply used a different solution (e.g. not use the phone for secret info or use a different, secure phone with open source software if necessary, etc.)