i think that bitcoin has two possibilities in two years roadmap:
a) value near around 3000-7000$
b) value between 0 and 100$
..and what will be? nobody knows
I pick a) but hard to believe Bitcoin would not reach $10,000 before stabilizing at $3,000-$7,000 within two years
i also think that second altcoin will be litecon... becaue bitecoin has one disadvantage - is not fast. i cant imagine the situation, that in restaurant i will pay with btc and then wait to 3 confirmations. this is the weakness of btc. so in my opinion is btc "gold" (fast and small transactions is not necessary) and litecon "silver"
next question is - what value would have usd in two years? inflation....
First of all you don't need 6 or 3 confirmations. Those are arbitrary numbers. 99.9% of time you 'll be fine with 1 especially for low amounts like a restaurant bill.
In any case for pos like starbacks even 0 confirmations will do the job. Of course some kids will try to take advantage and order a coffee and then double spend but it isn't worth the trouble anyway