I would have to say for me the biggest concerns for bitcoin would be..
1. The Darkweb gets shutdown or takes another hit. IMO the Darkweb/illegal activity plays a huge role in the success and failure of bitcoin.
2. Banks getting involved. If Banks are losing out on money or think bitcoin is taking money out of their pocket they could create a mess for bitcoin.
3. The richest of the rich will own most bitcoins. The rich usually make money from the middle/lower class, if they own most they will be no profits for them and bitcoin become obsolete.
Just a few of my concerns and obviously just my opinion. If you have any concerns or comments I'd like to hear your input.
To address your points.
1) That's no longer true. Yes BTC is used on the dark web but it barely affects the price of BTC. If the dark web were to shut down or switch over to another alt the price of BTC would only take a minor (temporary) hit.
2) How could that happen? BTC is decentralised. How could banks get involved?
3) That would never happen. The rich always want to get richer. That's just how things work. They know that if they own most of the BTC in circulation it will become pretty much worthless as nobody else has it to be able to spend it. For that reason it would make no sense for them to buy up most of the circulating BTCbecause it wouldresult in them losing a lot lot of money.