I think the first answer I'll give you is that they're still here, and still putting their money in.
When the owners first appeared there was no stadium site that had been identified and they were aware of how much work would need to be done to acquire one, and much of that work has now been done. It's a path that takes many twists and turns, with more surely to come, but I imagine they're aware of that.
The benefits are not as black and white as "stadium costs £75m, commercial development makes £100m = £25m profit" and that was never the plan. There is a benefit to owning and developing a club, with a good catchment and potential in the UK, especially one in a world famous place like Oxford.The aim is to build a sustainable football club and leave a legacy behind them for future generations.
These guys are major world stage players, and this is just one part of the jigsaw. You only have to see the word political leaders and business leaders that they mix with to realise their reach. They also genuinely like football, it's not merely business for them. I don't think leaving us in the lurch would help their profile with the powers that be in the UK. In fact I imagine having friends in high places in the UK may come in very handy once we get to the planning and consultation stage, particularly if the scheme is called in.
I meet them occasionally, when they travel over to games, but I don't know them personally beyond that. I have found them pleasant, polite, and genuinely interested and invested in our county and our club. I like and trust them, so I hope that is good enough to allay your fears.