I will reply to some of the points you made, not the entire proposal.
Downvotes on questions
If you want to remove downvotes, you must also remove upvotes. But voting on questions isn't useless. Votes tell others (including search engines) whether this question is worth reading. Some questions have good but misleading titles. Votes should be used to determine that. Too narrow questions can also receive downvotes, hiding them from the wider public.
More importantly, votes let the system know what the public thinks of it. It has two effects:
A negatively scoring question will be automatically removed if closed and is eligible for quick deletion via votes.
A negatively scoring question counts against the user in regard to question bans. A user asking useless questions should be stopped from wasting everyone's time.
Flagging as has"has no valuevalue"
We already have that, and it's precisely what you are complaining about that doesn't work. Questions can be downvoted, which indicates they have no value. Closed questions can be voted to be deleted. This system kind of works because we have millions of deleted questions, but it's not perfect. Why? Because we still get too many questions. People do not downvote enough and do not vote to delete old useless closed questions.
Replacing the current system with another one that will suffer from the exact same problem is not a solution.
Declining traffic
This was expected from the start. After all, there is only so much information one can add into the library. At some point new questions will be mostly garbage or duplicates.
The design of Stack Overflow implies the following flow:
A programmer runs into an issue.
They search the internet for a solution and can't find one.
They ask a new question on Stack Overflow
Someone (or themselves if they found an answer) provides a solution.
Others can add alternative solutions.
If a newer solution becomes available in future, it can be added as a new answer.
Developers who run into the same issue find this question during step 2 and upvote instead of asking a new one.
It was never the intention to allow users to keep asking the same question over and over. Stack Overflow did fail in informing users about it.
The result is that we have most of the topics already covered. Any new questions asked are either extremely low-quality pleas for personal assistance or duplicates of existing questions. Traffic on Stack Overflow decreases because people who search for the solutions online, find it quickly, which results in fewer new questions that can be answered. Fewer answerable questions means fewer answers. Traffic is dropping to the levels that more accurately show the search-results traffic.