Very proven, Yes. Holding bitcoin for longtime proves to be one of the best ways to grow money we have. Very much compared to interest given by bank.
Holding is no doubt a very good option for making a very good amount of money. If you have control over your nerves then this is the best thing which you should do. Holding of bitcoin gives you a guarantee of this thing that with each passing day, more amount of money will be added to your account. So you have to be very careful and you need to control yourselves especially at the hard times.
I don't think there's any guarantee about that at all. Bitcoin is a speculative asset and the price is built solely on the price. Bitcoin is a utilitarian thing and it is wanted for its usage or purposes. These purposes are already offered by other cryptocurrencies that are significantly cheaper than bitcoin. This means that bitcoin is built now by nothing but its hype and so are any other cryptocurrencies.
Since there is no guarantee about the price increase, then the investor has to be ready to lose the amount of money he is investing. One thing is clear about investments is that you are risking your money and there's no 100% guarantee profit in any investment. That's just too good to be true.
Holding bitcoin is a good way to make money but it's not a rise-only market. There will always be ups and downs and so far nothing is supporting bitcoin's market so it can go down at any given time.