There was a Reddit post a while back where they confirmed the OP was part of an A/B test to disable logging in to old.reddit.com on mobile. Not a great sign.
That makes sense though considering that they wanted to kill the 3rd party apps too. I'd imagine that for desktop users it shouldn't matter much wether you're using the redesign or old.reddit - they'll be able to collect the same data and show the same ads.
In January Reddit told third party app developers that there wouldn't be any API changes this year. Maybe in the future, but they definitely wouldn't charge for it this year.
Similarly, Spez said that old Reddit isn't going away.
To me, that's enough to consider it a realistic possibility.
Speculation, but it's an inevitability. It costs money to support old.reddit, which means Reddit will certainly kill it on the day that they'll think the backlash will be small enough.
To keep old reddit alive they need to also spend developer time supporting some new features and not breaking legacy code. It's more than just server costs.