fearsomepirate
Hero
I have a high suspicion that is a good majority of the sales and 5Es success is from the additional sources.. I think Hasbro wanted a reduced book schedule because for the profit margin RPGs make from print books (most RPGs don't make a lot of money) and they felt resources would be better spent elsewhere.
IIRC the reduced printing schedule idea came from Mearls or somebody else on his team. The reason was that constantly expanding the rule set is an unsustainable business model. Eventually, customers' appetite for more rules is satiated, and the expansions stop selling. The rules also introduce increasingly uncontrollable complexity, so you end up with crazy power builds where people cherry-pick features across many different books.
This path means rebooting the game completely every few years, which is just annoying, especially because the new rules by design must be incompatible with the old ones to get you buying again. You can't just reboot the splats; you have to rewrite the core. So even if the core is perfect, welp, gotta write a new core, because otherwise, people won't buy the new rules. When you're at the point of writing rules just to make people buy rules, you're just annoying your customers.
The sustainable model is working great for 5e. Their adventures sell well (better than any other RPG product on Amazon), and unlike rules, it doesn't gradually strangle your game to death with complication to keep buying them.