I love seeing the discussion here.
I see one of the largest motivations for proposing something like this is to experiment with QF approach to measure the impact in different forms. I personally also share lots of the concerns that projects like Rotki may not be able to compete with other projects with larger communities. Frankly, we (Gitcoin Holdings) have dismissed this idea (single round) for multiple rounds given this concern. This experiment likely would not have been proposed had it not been for Vitalik asking to see something like this for multiple rounds now - I am not saying this should be a reason for executive decision making, merely the fact that I respect Vitalik’s curiosity and desire to see the experiment unfold.
A few points to the concerns listed:
0 - Grants Round 12 is the first round we (Gitcoin Holdings) are working to decentralize operations of. As such, there will be great learnings as we proceed – timing on this discussion has always been shorter than we want (GR11 was even shorter!) 
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1 - There are no funds currently remaining in the matching pool (that I am aware of), that have been ear marked to a specific category. Keep in mind, Vitalik donated lots of dog tokens to fund public goods without an earmark for how those are used, and given the round size is now up to $1MM, the $150K from Kraken are long distributed.
2 - The QF mechanism has long tried to optimize for the many over the rich and few. This may be interpreted as a popularity contest, but it is also an incentivization mechanism for individuals with small amounts of money to have an outsize impact on the round. The true opportunity (IMO) is for increased curation mechanisms on the thousands of Grants we now have on platform. Going one step further, I am really excited to see the community ratify (hopefully soon) a policy on what criteria a Grant must adhere to to be eligible for matching funds. This is pretty loose right now (ie, no longer is it just digital public goods that are showing up on platform).
3 - As @lefterisjp mentioned, we have had Grants spanning multiple matching pools. ie the Uniswap Grants round and the Dapp Category round. I expect this to continue, and as such Gitcoin Holdings is working on optimizing the UX to make this more clear (for both funders and Grant owners).
All and all, I am supportive of the proposal under the condition that there will be an analysis after the round. This ensures there is documented learning, and continued conversations based on actual results, as opposed to just conjecture.