The logic is sound for a 60/40 IDLE/ETH in reference to slippage. However, I see the smart treasury as more of a primary market, as compared to the uniswap pool, and other exchanges, which is more of a secondary market. The secondary market is more optimised for slippage and has lower trading fees etc.
The Smart Treasury is more of a liquidity provider of last resort in this respect and should be therefore charging a higher fee to interact with (As time goes on; fees can start low and go up over time). The token-economics which occurs within the treasury will have a spillover effect into the secondary markets.
As fees from the idle protocol flow into the smart treasury there becomes an increasing price difference in the treasury price to the market price. An arbitrator can come along (when the opportunity cost is > $0) and interact with the treasury restoring the price to market price.
The good thing about a smart treasury is that if people don’t agree with this idea, the parameters of the pool can be changed over time by governance votes, such that the weights and trading fees are more optimised as a primary market for earning trading fees.
We are not diluting or inflating supply since no new tokens are created. And we are not releasing new supply into the market for frees (ie through a liquidity mining program), an investor must pay for the $IDLE we are investing into the treasury with ETH. Also, remember that governance could always reduce liquidity in the pool if they needed to fund community grants. As @Teo mentioned, however, there is still future discussions on how that mechanism should be implemented