I preferred the 12630s in the hopper to the 18650s in the TM2. It made it ultra portable with similar performance. Although mass production and immature EE design let them down. Everyone wanted a fatter hopper but failed to understand what that thing was doing with ~600mAh. 21700s would make more sense for sustained current draw, but you still get the best hits from freshly charged cells even with P42As. You might get a few more KMs on an ebike or EV from a big array. That's why they exist. For vaping it is overkill when the heater dynamic is intelligent.
Why are mains power adapters rare? Even MFLB had one and showed why it is better than batteries for vaping in a big way. I know of Lamart, MaxVapor. It should be much more common. I've thought about retrofitting the TM2. It would be so simple. And allow for external battery packs for those that want it.
I'm still swapping cells and externally chargng for my general use atm. Energy efficiency is what is missing from portable heavy hitters, not bigger batteries. It has been proved as viable.
21700s cost, availability, and the lacklustre EE on display is why thre are only a couple options and they aren't heavy hitters just barely portable and somewhat boring.