I think it works, and AWS has yet been brought down by this horizontal complexity. Quite an achievement, but might not be a satisfying experience for the engineers work there.
The downside is that a lot of their products lack polish which sucks. On the flip side even when they are launched with minimal features, they do tend to be reliable, durable and secure, which is important when it comes to data related services.
I wonder how widespread this view is. I suspect it's more widespread than Amazon realise. They may have optimised into a local maximum where they get a lot of value from being first to market, but could potentially get more by being first to "viable to trust a business on".
As far as being "viable to trust a business on" the numbers don't lie, AWS is number one because customers are running their businesses on AWS. The fact that DocumentDB launched with supporting quotes from Capital One, Dow Jones and WaPo shows that customers were clamoring to use it even before GA.
Remember a lot of these customers are coming to AWS because they tried doing themselves and stuggled. When it comes to data, customers trust AWS more than they trust themselves, and rightly so.
AWS also has not had a reputation for deprecating services it launches. I find very little risk in taking a dependency on something AWS releases.
They already are viable and trusted by multiple billion-dollar companies and governments.