I submit my code contributions, for free, because I am participating in a collaborative community effort called an Open Source Project. I do not typically contribute to the proprietary codebases of for-profit companies for free; I have a contractor rate for that.

If you say 'that makes it untenable for me to accept your contributions for free, then relicense to proprietary keeping those contributions', well, that's your problem. I don't particularly care about arranging tenable circumstances for you to sell my work under a proprietary license without paying me.

The way you accept external contributions for software without a CLA grant is by not attempting to take the project proprietary, and keeping the open source arrangement forever. I do not see how you could be confused about an open source project staying open source forever while taking open-source-only contributions. That is what almost all open source projects do.