I can see where you're coming from as I had to process those same thoughts. The lithium primary is still the gold standard for reliability IMO but a lot of reading and study has lead me to conclude that a properly maintained unprotected 18650 is very reliable. These are the same cells that make up laptop and power tool power cells as well as powering EVs.

The OWL is a bit heavy but the REIN is really no heavier than a Surefire. They also make a REIN Micro which uses an 18350 (IIRC) that's substantially smaller. Weight is always a factor, for sure. Moving my OWL over to the BREN 805 results in a real brick since the CZ weighs 8 pounds empty!

At any rate, I'm certainly not a salesman for Cloud Defensive nor are they the only game in town. Surefire does some "dual fuel" versions of their "pro" level WMLs that will run on lithium primaries or rechargeable which can be a good option.

Apropos of nothing I have been a lifelong booster of lithium primaries and a critic of rechargeables. But I must concede that over the last few years great strides have been made in terms of quality and affordability of the latter. Slowly I've been adding them to my tech stable and have been pretty pleased. As a long time skeptic of rechargeable batteries I have to admit that they're probably the wave of the future and even I can acknowledge their advantages in some applications.