The key network effect left is pretty much a leaderboard that fuels their daily retention. So any existing game company could potentially create another gamified learning app as part of their game pools.
I don't even think they think being compared to a game is a bad thing. What makes Duolingo the best in its field is the content and the varied exercises with a high quality.
But the exercises aren't varied or high quality. Try drilling a particular skill and you'll soon memorize all their sentences, to a degree that it trivializes the practice. Not to mention they're absolutely riddled with errors that strongly imply they don't even run the translations by a native English speaker. Voice exercises with numbers are simply broken in Russian. And all the comments are locked, with many of the complaints ignored.
There's nothing wrong with memorizing example sentences. Your brain is not a digital memory bank, memorizing is impossible without forming some deeper understanding on the fly.
And the memory thing - I don't have to think about what a sentence means. This is phrasebook teaching, a piss-poor method.