A whole host of questions that I feel are important. Any insights would be appreciated!

1. Can blockchain-based currencies really work with billions of users?
2. Will it become a choice between impractically expensive fees or impractically slow transaction confirmations?
3. Are the proposed plans for BTC and ETH scalability just short-term fixes that will eventually run into the same problems later on?
4. Are DAGs the solution?
5. How much time will IOTA transactions take if each user has to process their own share of the "tangle"? Can tiny IOT devices even handle such processing requirements?
6. Others have suggested that currencies such as DCR, PIVX or BURST solve the problem in different ways. If so, how?
7. Are there any other solutions?

Thanks