There aren't guarantees in life, but I think you're wholly misunderstanding the scale we're talking about. Visa processes about 200M transactions/day. Almost $17B/day. These are not small numbers.
I think you're also misunderstanding of how the financial system looks at payment rails - there's a pent up (ongoing?) demand for better ones.
Also, while merchant transactions are one market, they certainly aren't the only one. There's lots of talk about international remittances. This is a $500B+ market[1] that is growing at an insane rate[2]. Average fees are 9.3%[3] and way worse for some countries - it's not unheard of to say, pay $40 for sending $100 from the US to Kenya. Now that is a market that's ripe for disruption (you'd have to balance bitcoin inflows w/ capital flight for the economy to work, but it's worth pointing out there's already a service linking bitcoin w/ M-Pesa: http://motherboard.vice.com/blog/one-third-of-kenyans-now-ha... )
IMO, this is a particularly useful page which gives some context on where bitcoin currently sits with other payment networks: http://www.coinometrics.com/bitcoin/btix