Amazon's model isn't a failure per se, it's just not very profitable - that's the nature of competetive retail (where most of Amazon's revenue lies). My point is Amazon aren't skating by because they are over-spending because there is no evidence of this in financial reports, Amazon's low profits are endemic of existing only in low-end low-profit margin markets where volume is the only keep keeping them afloat and just barely. When you look the products Amazon have put out, particularly the Kindle Fire and Fire phone, their R&D spend is barely noticeable compared to their competition.
But Amazon have 154,000 employees (plus many contractors) and hundreds and hundreds of facilities around the globe with rent and utilities and those are some huge overheads. They still operate like a startup but that ave the infrastructure of a conglomerate. Amazon has 50k more employees than Apple and 25k more than Microsoft!
And this is the nature of a lot of industries these days, but when your margins are that low and you exist in a competitive market (which is every market Amazon exists in), a competitor biting down on pricing can wipe out profits as happens quite a lot for Amazon. If you tried to sell such a business model to bank (or investors) these days, you'd find it tough to get investment on a business model of 5% margins unless your volume is mammoth-sized.
Wall Street has tolerated Amazon because they have demonstrated that they can diversify into new markets and they have good and positive brand awareness. Wall Street hopes that Amazon will come up with some product/market they can capitalise on and bring their existing customers too but after 20 years, Wall Street's patience seems to be growing thin.