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yes miners have an incentive to protect the network. Essentially they are providing a service.

That's the whole point, Bitcoin is a closed system and because people are inherently greedy that greed is used to benefit the greater good.

If bitcoin was over provisioned then miners would capitulate. Which actually does happen from time to time.

Also you make an assumption that miners are only providing service per transaction. This assumption is wrong. Due to how merkle tree's work bitcoin makes it expensive to add blocks to the tree but cheap to verify the whole tree. Each new block protects and verifies all previous blocks and transactions.