I'm not surprised about them keeping both in the end, but I'm a bit surprised how early it was announced. I would've expected them to explore possible options a bit longer, especially from the point of view of pairing Hülkenberg with a young driver. Seems like they prefer to stick with the safe choice.
The only alternative would be to take on a youth academy driver from someone else for extra money or cheaper parts. But considering how expensive Mick ended up being for them, I guess Haas is vary of that option.