IMHO - redirecting inflation is the way to go on this.
The inflation method essentially creates a constant faucet which will trickle funds in at a predictable rate which can then be used to plan/budget (in STEEM values obviously, but not Fiat equivalents).
If the system were to rely on donations exclusively, the rate at which proposals could be funded would be complete unknown and donors would have to be solicited/engaged over time to make it work. That's an incredible amount of effort/politics, which could complicate things and ultimately lead to the failure of the WPS (e.g. donor donates to general fund, doesn't approve of what was funded with donation, refuses to donate again).
The source of the inflation should be the post rewards pool (author/curation rewards), since it makes up the bulk of inflation and has the most subjective value/impact on the platform.
RE: Pros and cons of two versions of Steem Proposal System