There are emerging cryptocoins which don't rely on the 'old' blockchain technology. The most known of them are IOTA and NANO (raiblocks).
I have never used these coins so far, but their developers state these coins can provide a lot of improvements and better features than traditional blockchain-based cryptocoins like Bitcoin. For example, faster and cheaper transactions, less energy consumption and so on. So do you think blockchain-based cryptocoins are doomed and the future will be favorable to these emerging technologies?
Neither Iota, or raiblocks have decent enough security models to challenge bitcoin. Iota is completely centralised and raiblocks cannot withstand bootstrap poisoning.
The true successor to bitcoin is yet to emerge, and the technology isn't quite there at the moment. It's an unsolved problem.
what do you think about Dagger (XDAG). They proclaim themselves as the first mineable DAG coin. I thought DAG was supposed to have no blocks.