I couldn't tell you the last time I bought something on Amazon for fiat and not for a voucher or gift card I had bought using bitcoin. Bitrefill is currently my go-to supplier. As well as being a quick and easy way to spend bitcoin, it has the added privacy bonus of not letting my credit card company monitor all my purchases.
Surely this would be the moment which would solidify mass adoption of the currency?
It might help adoption, but a single retailer accepting bitcoin, even one as ubiquitous as Amazon, is not going to catalyze mass adoption on its own. Maybe bitcoin users who also use Amazon (which will be the majority of bitcoin users, because the vast majority of people in countries where Amazon operates use Amazon) would start using bitcoin instead of fiat when shopping at Amazon, but it's unlikely to cause huge numbers of people to get involved in bitcoin for the first time. As long as Amazon continue to accept fiat payments, then accepting bitcoin alongside offers little incentive to try out bitcoin for the first time. We also need more than just Amazon to accept bitcoin, since they would likely use a payment processor who sells all the bitcoin they accept immediately for fiat. We need a whole ecosystem of bitcoin, with employees at Amazon opting to be paid in bitcoin because they have plenty of available merchants or retailers at which they can spend it.