Why is it so unbelievable? As long as bitcoin gains mainstream adoption, 1mil (of purchasing power) per bitcoin is inevitable.
1. The ~10 minute confirmation time
2. The reliance on set encryption protocols which could be broken in the future (EC and SHA256). For something to become as big as a world currency, it needs to be trusted that the technology would last hundreds of years.
3. The blockchain size.
4. The amount of resources that the client uses on a device.
5. Other scalability issues, such as the amount of transactions which can be handled per minute.
6. Irreversibility of transactions.
7. The extreme ease of the average person losing coins due one of many vectors of mistakes and hardware failures, or hackers.
8. The technical difficulty for the average user trying to even use it.
9. Bitcoin commerce and usage has mostly formed within centralized entities even though it is supposed to be decentralized.
10. There are still humans who ultimately control Bitcoin and who must be trusted by its users, even though the system is supposed to be decentralized.
11. Volatility - it is not an attractive store of value for someone who just wants to use it as a store of value or a currency and isn't into investing or speculating.
12. The present distribution of wealth.
13. The reliance on the internet in order to function.
14. Also, what happens if the internet is split apart due to government firewalls.
15. Governments, corporations, and hackers could ultimate control and restrict Bitcoin usage if they really wanted. And mass adoption will not be reached when users have to do it illegally, run tor, and run checks on every client they download to make sure it isn't a hacked client. Even tor traffic or bitcoin traffic over tor could be blocked too by smart routers.
16. Some of the worlds most powerful contries have already banned or restricted it.
** add more here ** - anyone else is welcome
Thank you for the objections. Far better than the morons that hate btc cause of no reason at all.
1. transferring coins from one coinbase account to another is instant. In the future there will be many companies that let you pay for things instantly without touching the blockchain. Sounds like right now with things like visa/mastercard? Yes, but now most of your longterm funds will be in bitcoin addresses while short-terms funds will be given to places like coinbase for purchases. A 10-min wait to transfer moderate amounts of savings into spending money is not an issue. (If you have not seen the benefit, this means most of your money is safe and in your control, and can't be inflated away or stolen/frozen)
2. "which could be broken" aka not an issue. Until you have proof, I don't even see this happening.
3. Will never be an issue. Computer memory is going to outpace the blockchain size for a long time to come. AKA computer memory is going to double every 2 years like it has been doing.
4. non-existent you mean?
? Internet browsers use more than the clients I think 
(do not quote me on that)5. See reason 1. Were supposed to by design. Micro transactions will be done by visa-like companys, while moderate amounts of bitcoin movement will have to pay for the privilege.
6. escrow/not an issue, learn to double check the address b4 sending.
7. There is big money on solving issues. For the person unable to take care of their coins, you can pay someone to make sure that those things do not happen.
8. Solvable, see Neobee.
9. "supposed to"? says who? Bitcoin protocol is the only thing that was advertised and is still, in fact decentralized.
10. Who? client developers? They are open source, and do not need to be trusted. If they make closed source clients, then you can use the old ones, and I am sure if they fall from grace, there are many developers on the sidelines just waiting for the opportunity to make a name for themselves.
11. Yes, having your savings skyrocket in value about once a year is terrible
. I have never understood why up/up/up volatility is so bad. Anyway, If it really is sooooo scary, then wait until it becomes more popular. The bigger the market cap, the less volatility12. Personally I don't consider that even an issue. If other people having more money than you is a problem, there is no cure. As long as wealth exists, whether it is in bitcoin/gold/fiat/physical possessions, there will always be wealth disparity.
13. The internet is growing every day, and is not going away. Internet is going to be a worldwide thing in ... what... 10 years?
14. Bitcoin is the least of our worries in that doomsday scenario.
15. How would they? The government is not as uber powerful as you think they are. Pirate bay is still going strong.
16. And others have not. As long as some countries do not, or at least don't have ways to restrict its use, than it can still thrive. Granted, It would slow down mainstream adoption, but I do not think it would kill eventual adoption.
oooook, So I admit, In doomsday scenarios as giant internet firewalls worldwide or all governments banning bitcoin, and actually somehow enforcing such draconian laws, then yes, There is a good chance that bitcoin won't be reaching mainstream adoption. But I believe those scenarios to be far less likely than bitcoin reaching one mil per coin.