a product is lost or disapear due to be replaced by something newer and sophisticated maybe it will happen in bitcoin when bitcoin get a very low price example 1$ the user may start leaving bitcoin and use new cryptocurrency or money online is becoming a breakthrough nicer than bitcoin

Probably not do anything differently at all. If it drops I just say, "Hey it will rise up again in the future."
When it rises then I am little more concerned because then I say, "Damn why didn't I buy some yesterday when it was cheaper!".
I usually end up saying the latter most of the time.

If bitcoin drops to 1 dollar, ill buy more and invest it till the price increase

No matter how bad a certain situation may look, there is always an opportunity to make a lot money in quite a short time. As you also said, if you buy at $1 and sell at $2 you'll double your money without much problems. I think I would definitely buy myself around 10,000 BTC priced at $1 each. If the price doubles I'll sell 5000 BTC and take my $10,000 back. Then I have 5000 BTC left which I will hold for a very long time to see where it goes.

I will believe if there are another people will say to think twice, for to do that, because it seems like the coin in front of their hole of dead and in my minds investing on them is between alive or dead.

I hope that when the price drops to $1 I wouldn't have any bitcoin on my wallet, what would I do is investing $xx in bitcoin and leave them on a wallet for few years but depending what will happen bitcoin has always a potential to have higher and higher price possibility.

Don’t know what will I do because I am not able to think that this could ever happen.If that happens then my trust on bitcoin will loose a little.But still I will but 100-200 BTC atleast and then wait and see what happens next.

I'd continue to use it unless there was a fundamental flaw with Bitcoin, although if it did reach that it would likely be due to an unrecoverable flaw.