Can basic price of a game go down on Steam?

Yes.

Yes, but it'll take a long time.

Yes

Do you think Assassin's Creed II was $20 when it came out?

No, why would you think that?

Games used to only be $19.99, then Steam started charging $59.99 since they almost went bankrupt.

Fortix was £5.99 in a time gone by.

Of course it can, even Metro 2033 and the Ship were at full price when they came out.

Yes.

http://www.steamprices.com/eu/track/25/list (though there are some errors in the list)

of course they can. for example max payne 3 price dropped recently by 20 euro. It depend from company how fast price go down. Bethesda keeps prices same for long time after release, while thq or square enix lower then really fast.

Sure, else you would only see games that cost $60. It depends on the developer/publisher to make the price.

Yes!

fortix is a prime example of price drop

Yes, in a few years time :p