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?
The money supply roughly doubles every year, then every three years it is reduced to 1/10. This keeps the coin count numbers approximately within the same range longer term, but doesn't affect much else.