I'm just talking LEDs here.
When Luxeons first came out, they weren't available in many production lights. There wasn't nearly as much of a market for LEDs, because the output was generally so low. Early mods upgrading greenie P bins to Q3J and R2H emitters, boosting stock circuits to 700mA instead of the 350mA spec'd for 1w LEDs...we made big differences in the performance of our lights. As high domes and higher flux bins became available, we could still make a noticeable difference by swapping to premium tints, ditching optics in favor of reflectors.
The expensive ones were lights that used custom parts, which became popular. Many of us wanted something nice and unique, not just a cranked up factory light limited by its design.
Buck/boost circuits let us use Li-ion cells that hadn't yet taken off enough for companies that were building lights to consider in their designs, too.
Now you can buy a $50-75 light that will smoke the ones we were spending $2-300 to build a couple of years ago, and flashlight manufacturers are incorporating multiple modes, adaptability to various battery configurations...things we could never get out of the box back then.
Now there's not much we can do that's practical to make a drastic improvement, except maybe improve the tint, and now manufacturers are catching on to that, too.
'Course there will always be people looking for more and more options and output "because they can", and mods like some that we're seeing with P7s and MC-Es(or multiples of those) will doubtless continue, but that's at a whole 'nother $$ level than what most of us are willing or able to spend for a EDC light.
I'd like to do a few more mods, but they'll be on an old favorite or two that I enjoy, and would like to update.
Most of the time, now, there's no point in doing them when we can get what we want straight from the manufacturer, or from a builder like Malkoff who gives us so much for so little investment.
As in many other aspects of day-to-day living, I'm sure the trend toward instant gratification
has had an effect on the situation as well.
A fact of life when you start modding, beyond expense, is time and aggravation. If you don't get a viable return on those...how long are you going to keep doing it?