It is gaining mainstream credibility thats already an enough reason for you to invest in cryptocurrency. I dont really think the last point is making any sense at all. Yes there is a limited amount if Bitcoin, which is 21 million, but that doesnt mean that it wont be enough
Nobody invests with the intention of holding their Bitcoin forever, you will always sell to make profit sometimes or maybe for some other reasons. By the way, investment is not even meant for everyone, its all about choice and those who wants to be into it.
It is like a chain continuously going on since the first day of bitcoins till now. Like you have said, there is fixed number of bitcoins and that is twenty millions in all over the world. With such limited resources against the whole population of world, these coins are continually sold and bought so the chain is going on without any interruption. Just be hopeful that someday bitcoins would be increased.
Before investing into something, I usually spend so much times to find the reason can make which thing I want to invest increase the value in the future. For Bitcoin, I think Bitcoin can changes its system to faster than currently with low transaction fees is enough. It only has this problem to make its system is still worse than if you compare the Bitcoin system to new system like Ripple, Ethereum.