"Petition" to persuade Emisar (Hank Wang) to make a larger version of D18

On the contrary, it's for your safety instead (the customer) that the manufacturers do that. It's almost important that very high powered flashlights like the X50 or MK38 have a built-in battery pack to properly and safely handle the charge and huge discharge requirements, as well as cell balancing and so on.

Tactical_grizzly has done a good teardown of a X50 battery pack: https://budgetlightforum.com/t/-/69378

For a variety of reasons, having such a high powered flashlight with individual user-replaceable cells becomes very problematic. There is a discussion of some of the issues on my MK38 thread. https://budgetlightforum.com/t/-/69119 . Even flashlights such as the D18 which puts 3 user-replaceable high-powered cells in parallel is arguably (and I know an unpopular opinion) borderline dangerous as a consumer product (but yes I understand that it's fine as an enthusiast item). Any accident caused by improper use of the flashlight, even by the user, can be a big liability for the company since it is their responsibility to engineer a safe product.

It's definitely possible to design a decently powerful flashlight which accepts 3 individual cells, and has all the appropriate electro-mechanical protection, and also supports USB C charging and powerbank functionality, but you'll be looking at a $200-300 product at that point, not close to the $100 price-point of the D18 or similar. The MK38 comes in a user-replaceable version with no USB C charging, but I do hope it has the right protection inside. If you compare the integrated and non-integrated versions of the MK38, an extra $60 gets you a completely different machined body, a decently implemented USB charge and powerbank circuit, 3 very decent 21700 cells, and (hopefully, I haven't verified this yet, but high likelyhood) an integrated BMS. For a product that likely sells only in the hundreds, the profit margins are likely not very high and I think it represents OK value.