The nicotine availability is the choke point, take away the nic and vaping is banned. Take away flavors from the retailers and they will dry up (exit in FDA speak). The sales on e-liquids is where the profit margin is for the vendors. DIY costs about .05 per mL to make, the vendors can sell it up to .50 to $1.00 per mL, far higher profit margin than hardware.
I would shudder to think my nicotine suppliers were surviving on the bottle I buy every 6 to 9 months for DIY.
Running an online business doesn't cost much. It's the B&Ms who really need the e-liquid margins. DIY is still scary to a lot of folks. Based on my own limited experience w/ JC, I can also say that it's not that easy to come up with somthing comparable to what the experts can do. And it took me a good 6-8 weeks before I was willing to try DIY.
B&Ms have been hugely successful in increasing the pool of vapers, by providing the tasting tables and the equipment advice. Most experienced vapers speak in shorthand which isn't that easy to understand for noobs. Despite the efforts of people like Grimm Green and Phil Busardo (and others) to try to teach "Vaping 101," getting beyond cigAlikes is still difficult for someone who doesn't have a local helpful B&M. Four months ago, one guy in my local B&M gave me a few pointers. Now I probably know more than he does (he's not an ex-smoker, nor a particularly avid vaper: just a good retail businessperson who knows his products).
I have to confess that my own record of converting the smokers whom I know has been pretty poor. I like to think that I could've done better with a tasting table, etc. Although I had an uphill battle, since their concept of vaping was too influenced by their experience with cigAlikes.
Once the special-purpose vape shops die off, that's going to mark the end of vaping-as-cessation. Most new vapers will be dual users. This will in turn allow the CDC to accumulate the needed data to fully justify the argument that vaping encourages "more smoking, not less." Not that they ever needed or cared about good science in this context. But it will be there, when the time comes to move towards full "medicaliation". By then, any remaining non-BT cigAlike co.s will be absorbed into BT, and things will be back to where they were four or five years ago in the US.
I feel as if I'm one of the last people in the door, so-to-speak. Although I'm sure there will be at least another few hudnred thousand who get in behind me, before it slams shut.