If 1/10 of the supply is being wiped every 3 years then we have a problem cuz SP tokens will only be backed by 0.1 steem.
This seems like a misreading of some aspect of the economics involved. What are you basing this off of?
Whitepaper says : In order to compensate for the ever increasing precision, the STEEM network performs a 10:1 reverse split every 32,000,000 blocks (about 3 years).
When they reached about 5b coins which is about 32 000 000 blocks the split will occur. After that SP tokens will be backed by 0.1 steem since 1/10 have been wiped.
The VEST to STEEM conversion rate will adjust, and the amount of SP will reflect that...