What about stakeholders that don't make posts or comment? Should they just ride on the coattails of creators and witnesses? What about decreasing the ability of witnesses to secure the blockchain, or decreasing their incentive to do so? Why base the contributions of witnesses on their rankings rather than their stake?
If we don't get some donations at the get go, I don't think we'll be able to start funding things soon enough, so some donations are probably necessary. Even so, I do think you're right that donations alone won't meet our needs in the long run, or even the foreseeable future. However, what we fund through tax, and what we fund through donation - which will always be possible, no matter if we tax or not - will matter alot.
How we choose what to develop, and how much to fund that development, is almost a larger question than how we generate the funds.