Not having to take the light off the bike and charge it and then forget to take it back to the bike, not to mention forgetting charging it and finding out when it's dark, is completely worth having a dynamo.
"This showed that for cars DRL reduces the number of daytime injury crashes by 3-12%. The effect on fatal crashes can be estimated as somewhat greater (-15%)."
https://swov.nl/system/files/publication-downloads/fs_drl_ar...
This is about cars/motorcycles and daytime, but it certainly applies to any moving vehicle at any time...
When driving, I love those bicyclists that have a blinking rear light btw. Can't overlook them.
If you're worried about being without light, a (typical) dynamo system is more complicated and exposed than a battery system, so will be more prone to failure.
If you commute on a daily basis, a hub dynamo and light system is a bliss. Just hop on the bike and go. I have used bikes with Shimano, SP and Son for thousands of kms in all kind of weather and never really experienced a fault. It’s as simple as car lights - you just take them for granted.
With battery powered lights you need to take them off and put them back; recharge them; remember to bring them with you and not lose them. A spare battery pack is not enough (front and rear) and may not work during cycling (not all lights can be charged while turned on). And, low quality battery powered lights tend to quickly break (2-3 years) while I now realize one of my b+m systems is 10y old already. Good battery powered lights will probably last more, but they’re as expensive as dynamo powered ones.
So yeah, battery is ok and cheap for casual cycling, but very suboptimal if you want reliable lights every day throughout the year.
Rechargeable lights from the usual suspects are generally not good, they are expensive for what they are, have low capacity, and don't have swappable standard-size batteries.
They make dynamo systems look like a good deal, but if typical battery-powered lights were even close to their theoretical optimum I think people would be much less enthusiastic about dynamos.
Ofc if you mount fixed battery-powered lights and you could just swap a usb c battery, maybe it would compete with dynamo. But an easy swappable battery would still ve easy to steal (unless it’s inside the frame with a lock or sth like that)
Hub dynamos seem a bit more fragile, with a wire extending into the lightbulb, but never heard reliability is a concern with it...
A spare battery in your saddle-pack