Maybe they needed the staff to build the software back then and now they could be leaner cos the product is already built. Maintaining is much easier than building.
In all honesty, it doesn't seem like 400 people were really needed to build the software. If you just look at credits in Adobe Premiere, for example, there's not 400 people in there, and it's a way more complex product than what Evernote is/was.
