Interesting discussion. And one that's not likely to dissipate anytime soon.
Here are two basic points to consider:
1) Everyone who paints will be represented in the Postcard set's Artist Bio Cards, whether it's 5 or 50.
2) Limiting the number of postcard sets is a decision Topps made based on customer feedback. It the only way to ensure that most people that want sets, get sets.
Given these two things, there's no way to make any guarantees for completing a set of artist cards. But at least, it encourages trading and hopefully, the people with extras will offer these at a reasonable price to forum members. We have to rely on goodwill here.
On a related topic, I've given lots of thought to the best way to offer postcard sets, one of them being an advance notice to mailing list members. The problem with this idea is that anyone who wants to join the mailing list is welcome, whether they're collectors, dealers or something in between. So, unfortunately, we'll probably always have those people who try to flip a bunch of sets. In my opinion, the only way to control it is to block those individuals caught buying more than the allotted number of sets (no easy feat, since most buyers have family and friends they can ask). Someday,we may find the perfect balance but for now, we have to rely on collectors to report anyone (that they can prove!) is abusing the system.
As always, I welcome your suggestions and I'll be reporting on the upcoming set soon -- hopefully, before Halloween!