Let's go for 5k$ as it sounds pretty realistic (outside bubbles).
Biggest threats will come from Ethereum or better cryptocurrencies that will scale faster compared to Bitcoin and all its forking drama.
Invest moderately, no need to rush.
I don't think so. Maybe Bitcoin will be 1k.
I think that because you never know what can happen.
Maybe there will be another coin that's more valuable. The future is always unexpectedly.
If the user amount increased from 1 million to 10 million, the price will increase 100 times. So it will be 100*400= $40,000.