I think there are several reasons:
1. Quantum computers will make miners lose their jobs because quantum computers have tremendous cryptocurrency exploits.
2. The crypto will not be supported by dev, they will fail and lose value.
3. These SCAM ICOs will make investors withdraw money from cryptocurrency because they think the risk is too great.
4. Bump, dump games will ruin the market.
1, miner is the game for reach people who has long-free money to invest, so they would never die when quantum computer come out. and even more, quantum computer still so far away till the day it can beat miners's computer
2+3, good investor will never die anyway. they always have safe money that can guarantee even they lose all others, they would still healthy alive. and some crypto currentcy with strong dev alive
4, pump and dump game is just temporally. no one control market whole the time
Your explanations seem reasonable but we need to think about the long term. I have seen a lot of cryptocurrency on coinmarketcap.com and found that there is a lot of junk. The quantum computer I think will be born soon. I have read some articles about it. The market will continue to grow in 2018. Currently, only momentum for growth.