The best way to be able to gain Bitcoins even if you don’t buy them is to do trading. If you can sell them on a higher price, you can earn profit out of it.

Based from my own prediction, I guess it can’t be that super long-term that lasts for 100 years because sooner or later, tangible objects will be prioritized.

Based from the current standing of Bitcoin, I don't think it would face destruction sooner or later. Besides, millions of people are using it today.

The mode of payment has already been evolving. Bitcoin is part of the evolution and currencies have been changing ever since people have learned to trade and transact.

It’s tricky to tell since Bitcoins can be very volatile. However it does have the potential to absorb  the existing finances in various parts of the globe.

If you were to use graphics cards to mine bitcoins, you would immediately see that it is simply not profitable. If you are planning to mine for bitcoins either do it with a specialized hardware like ASICs or just go for cloud mining.