Apple won't have extendable screens until rollable screens are reliable, good quality, durable and don't increase the device size too much.
Foldable phones are a stopgap, and the current implementations poor at best. Nice gimmick, nice tech demo, but ultimately too impractical to replace normal smartphones. Not to mention the price, which allows you to buy the 10.5" Air, as well as a flagship phone of your choosing above base spec and still have money left for a set of wireless earphones.
What's the timeframe on rollable screens?