I believe, for the reasons that you outline, ETH will in fact remain the dominant collateral for MCD by design — calls for this are already being made:

“MakerDAO, in part, ensures Ether’s value by keeping it off secondary markets.

When Ethereum moves to Proof-of-Stake, the security of the network is directly tied to the value of Ether. Ether must be valuable, for Ethereum to be secure. Because 1 DAI = $2.5 of Ether, DAI strongly contributes to the value of Ether, and therefore the security of PoS Ethereum.

As new assets come to Multi-Collateral DAI, Ethereum will begin to lose share of the collateral that backs DAI.

I propose a few guidelines for consideration, as MakerDAO proceeds with Multi-Collateral DAI:

-1 DAI should have minimum 1$ of Ether collateralized
-Ether should be the most significant source of collateral: Because MakerDAO is on-top of Ethereum, it requires Ethereum to be secure for it’s own well being.”

https://medium.com/@TrustlessState/the-role-of-ether-in-multi-collateral-dai-cfc8b43ab50c