In the video that I posted with Geoff Keighly interviewing the developers they said that because of the time involved everyone was scared to do anything Half Life related and the only way they were going to do something is if it was different/unique. They experimented for a while with different things and started working on HL:A in 2016. It took a while before they were confident in it to even push forward with development. During playtesting which was only a 15 minute demo, they often had people playing for upwards of an hour just exploring doing random things that you wouldn't even consider in a 2D game.
They also discussed about the reasons for going VR rather than 2D with one of the explanations pretty much summing up what VR brings to a game. In a 2D for example, you come to a door, you press a button and it opens. It has 2 states, open/closed. In VR you can interact with it just like you would for real. Open it slightly, peak through the gap, lob a grenade in, shut the door again etc. Something that would be impossible to do with a 2D game and traditional controls.