I agree that batteries will probably be a limiting factor for an old light. It will be like trying to get 5.25" floppies for your old computer, or new music releases in an 8 track format, etc.
Breakthroughs in battery design will most likely occur, especially given the push for solar energy storage solutions, electric cars, etc. Eventually, a point where a very small battery can provide a lot more power will be crossed. Circuit breakthroughs will eveentually allow smaller and smaller placements, with less heat.
The industry will then naturally be able to make extremely small and powerful lights, and, extremely powerful larger lights.
It will be analogous to computer sizes, where the first high performance ones were essentially rooms full of vacuum tubes and tape drives...and now you can get something that fits in your pocket with more computing power.
Form factors for batteries may actually preserve some old standards, such as the AA, etc...due to conformity, especially at the joe consumer level. The shopping cart purchaser will not want a light that takes "some weird size", and may in fact gravitate towards disposable lights, especially if technology makes them more powerful and long lasting than they are currently.
If new technology develops amorphous batteries, such that its cheap and easy to mold the battery into any desired shape or size, freeing the industry from "standard sizes" that help with marketing and cost controls, then flashlights for example could take on new form factors: The battery could BE the light, etc.
So, just like I have an old Mac IIsi from the 80's that still works, and does what it did back then just fine....I don't really USE it the way I used to, as the newer versions just do the same things BETTER.