The "cost" of user creation is actually nearly nothing, fees currently charged for it are a spam reduction mechanism. Ease of account creation is being addressed in the next hard fork.
As for how SMTs will reach a billion users, consider this. Currently only users of steemit.com and a few other sites have access to steem blockchain, while with SMTs anyone can create a custom token, do an ICO and integrate into their business. Users of that token may never need to go to steemit.com at all.
Basically SMTs allow people to take Steem blockchain with them to whatever they are doing instead of having to come to steemit.com to use it.
Steem blockchain will now come to them and become integrated into websites and businesses they already interact with.
There is hardly a chance for a billion steemit.com users, (who needs another facebook?), there is a good chance for a billion+ steem blockchain users because they will use it with sites, programs, games, businesses, .. whatever, in the form of SMTs.