As the technology is rapidly changing, I am replacing LEDs every couple of years. However, the cost of the LEDs are dropping just as rapidly so keeping up with increased output, better optics, better battery technology, etc. isn't all the much of a burden. At $200 per light, I wouldn't be replacing the lights all that often. At $100 per light, I'd still think long and hard about the next big thing. At $25 per light, what the heck?

Yes there are some advantages to dynamo lights. I've never said there aren't. And, yes, I've had batteries run out of power. But battery lights aren't all that unreliable. I can think of several scenarios where a dynamo has failings as well. Snag a wire on something and you don't have a light. Ride off-road and you may not have enough speed to keep the light running. Have to walk a tricky section off-road and you definitely won't have light which is a bad time to be in the dark. Crash and break the light off the mount and you're sunk. There are lots of ways that you can have a single light fail and leave you stuck.

I run a single light in the morning this time of year but that's the only time I do so. I have redundancy up the ying yang because I know, from very long experience, that things can go wrong with any system. I even have back ups for the back ups. And I've had to resort to the backups (not the backups for the backups yet but it's only a matter of time). One would think that it is next to impossible to have 3 lamps fail but it can happen. I still have the lump on my thigh from the crash to remind me. Given the energy of the crash and the hurling about that I got, I doubt that any lamp...battery or dynamo...would have survived intact.

Your supposition is wrong. I'm not trying to optimize only money. I try to balance money, convenience and output. I always have. If the current crop of LED lights cost $100, I wouldn't suggest them to people looking for a light. I can build a system that puts out that much light for a lot less. In light of the fact that you can get 500 to 600 lumens for $25, I wouldn't suggest the $45 Planet Bike either. It's just not worth the money. Also in light of the cost of the current crop of LEDs, I couldn't answer the question "What light should I get?" with a dynamo system. Sure it's convenient for a single bike and you don't have to charge batteries but it's also nearly 10 times the cost for a whole lot less light. And, if you have multiple bikes, it's not all the convenient.

I'm not trying to change your mind, just as you can't change mine. I was only using your example as an illustration of the cost differences. Let's not forget Post #1 and benze's original question.