We're in this exact same situation in my company. A couple of engineers gave notice in the last week complaining about boredom. They ended up convincing one to

We're in this exact same situation in my company. A couple of engineers gave notice in the last week complaining about boredom. They ended up convincing one to stay(salary+ & better projects) but I feel their pain. I think what happens in a start-up is that once the company reaches a certain size, the 'hard-part' is already done. The type of engineer that gets attracted to working at a start-up is usually one that likes to be in over their head a bit and trying to solve hard problems. Once that 'problem' is basically solved unless they move on to other things(platforms/frameworks/languages/etc), they're inevitably going to get bored, complain, hate their life and then quit. I'm forwarding this article to management here, hopefully they'll get the hint.