That might be true for a small percent of HackerNews readers that are also Daringfireball readers - but the world at large still trusts Amazon; that's the whole point of the article, really.
As anecdotal evidence, an Amazon Investment might be the only way to convince me to become a Prime member (if that'd be a required precondition). I'd almost certainly use it, if allowed.
Unless you're paying close attention, it's not obvious to the average consumer that so many products are being sold by independent resellers vs Amazon themselves, so people aren't as careful as they might be on, say, eBay.
But... I just don't see how amazon would get me away, say, from vanguard. There are already good retail brokerages, and I don't see why I'd want to go through amazon instead of another company.
A. Going global: a lot of those "good retail brokers" are US-only. Sometimes it's dumb - I already have an ETrade account & signed all the paperwork including stuff relevant for my foreigner status... But ETrade will only let me trade my employer's stock, nothing else.
B. Bundle stuff that others can't match - e.g. Prime
C. One account, with good actual security (few appreciate that but some will)
D. Maybe new financial products that others can't match (I think Amazon is in a position where they might pull that off)
As for Amazon, I would expect them to eventually expand globally if they were to offer such services (which is not what I expect from most US brokers).
but B? I don't see why I want to bundle my stocks with my television series. And, I'm not sure Amazon has security that is any better than Vanguard.
[1] E.g. they obviously run stuff on AWS: http://www.vanguardjobs.com/job/7980466/ts03-cloud-architect...