Hm. Money may not have been considered in the original design, but motivated attackers were. A volunteer platform is different to a monetized platform, but I don't think introducing money changes the threat model for existing components, or at least not in the way you seem to think it would.
The question is, motivated to do which attack? I can understand protection of the secrecy of the data. But without consideration of money, I don't see how protection against faking work would be up to snuff.
But as I said before, I know very little about Tahoe-LAFS. I described the kind of issues it will have to deal with, if it's up to the task, more power to it.