1. miners cant spend their earnings for 100 blocks

2. anyone who has had a recent transaction has incentive for blocks to continue being produced to alleviate risks of a double spend from chain rewrite

3. hash rate needs to be used.. related to #2, if its used for a shadow chain to rewrite further back it can be quite destructive.

4. yes transaction fees.. and if theres low/no transactions then space will inevitably be used for inscriptions and op_return messages from miners as a permissionless data distribution network.