Re: 10 BTC. Will I be a millionaire by 2030?

In the long term, the bitcoin price will rise due to mass adoption. In 14 years, the bitcoin price could be $200,000.

I think you are pretty conservative here.
Despite the adaption rate will decline when the majority of the masses have adopted bitcoin as their currency, the inflation is also getting lower. Therefore I think 10 000 % increase after each blockhalving is actually quite realistic. That means, the current blockhalving should result price of around 60 000 usd/btc sometime in the coming years. The block halving of 2020 will make the price to millions of dollars per bitcoin so 2030 holding even 0.1 btc should be able to make you a millionaire not to mention of holding 10 btc.
The advantage of owning more bitcoins is sooner millionaire status.

I don't see how the inflation rate decrease once more people use to give demand on coin, isn't it should be the other way around? Inflation should increase fast because it will be use for more users.

In fiat system the inflation should increase with the increase in economic growth because higher GDP requires more money.
The beauty with bitcoin is that it is tightly limited to 21 million coins. When bitcoin economy grows, more coins are needed and the holders need to sell but if they are like me it has to be more expensive to make me selling even a small amount of coins. Therefore Bitcoin is like a pyramid scheme where the new comers will make the price to rise and the existing holders will gain if they buy and hold the longest time. The best thing is that the new comers do not need to worry about at what price they will pay for their bitcoin - it still is a bitcoin and a part of the global digital currency pie.