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

1 = No. Block chains can't scale enough, even with an "unlimited" block size they will still hit a limit. Only solution to that limit then is semi / full centralization.
2 = There is no choice with block chains, they just can not scale enough.
3 = Yes, on both counts.
4 = DAGs are a better medium term solution, but these too will have their own set of different problems eventually.
5 = I don't know enough about IOTA specifically so I can't (and wouldn't like to) answer.
6 = If its block chain based and the project is claiming scalability in the 10,000+ then they are either a) lying b) semi / fully centralized c) both
7 = Yes...There are a couple of technologies being developed but they aren't ready yet. You should start to see these 3.0 platforms in 6-18 months