This is not a critical problem (or even a problem at all, depending on whom you ask). This has been argued again and again, and what is always comes down to is that, according to the Keynesians, an inflationary currency is preferable for encouraging economic growth, but the reality is that A) economics is not hard science and B) as long as the inflation/deflation rate isn't extreme, both inflationary and deflationary goods make fine exchange mechanisms.
>supply of the coins need to gradually increase to support the flow of spending.
Why? If adoption increases, the value goes up, and people spend fewer bitcoins for the same thing. Bitcoin is especially good at this because it has a lot of base units.
>They will sell their BT for currency rather buy depreciating goods with them.
This is incorrect. You're arguing the extreme of the effects of deflation. People will buy goods with deflationary assets, because sometimes the expected value of owning a good is higher than the expected value of keeping an equivalent value of the deflationary asset.
The equivalent (and equally incorrect) ad extremum argument against inflation is "With inflationary currencies, no one will bother working, because they know the payoff of their labor will be worth less by the time they get it."
Bitcoin doesn't need to "support a global economy" for it to be useful/successful.
"Right now more half of Bitcoin supply is already own by someone"
Are you saying one entity owns half of all Bitcoins? I've heard the figure of a few percent thrown around, but never half.
But that's all about inflation in the long term. In the short term, the rate of inflation depends on other factors such as the rate of capital moving into the Bitcoin economy and the rate of Bitcoins being lost.
So right now, there's no reason people should hold back on adopting Bitcoin because of its current monetary policy of having a fixed maximum number of coins. It's not like there's a fixed amount of money or time that can be invested in it; there's no reason people like Stripe and merchants can't do their bit at the same time core Bitcoin developers do theirs..
My economic activity in that currency would enrich others who have done nothing but havng on to it. Not really an attractive feature.
Actually it's current inflationary and will be for a while. It's deflationary eventual nature is a long term issue with plenty of time to be addressed; short term, it's inflationary.