There's always going to be a robust used market for phones that were purchased outright, to be resold on a different cycle than every 2 years (plenty of rich people changing phones every year, and plenty of people replacing on a 3, 4, 5, or 6 year cycle). You can expect the market to basically settle on a curve where it depreciates along a predictable rate.

Leases don't really change that, any more than leases changed the market for used cars, or even certified pre-owned by the same dealers and organized by the same manufacturers who sell new cars.

There will be times that the predefined lease terms will unexpectedly prove to be either beneficial or detrimental to the consumer. Sometimes external factors will affect the entire used market, like currency issues, or component pricing issues (imagine if RAM prices dramatically swing again for new devices in a way that affects the value of the already-sold devices out in the world), where the predefined lease prices turn into a windfall for someone. Like in 2021 or so when expiring car leases allows the lessee to buy out the car at the end of the lease for much cheaper than the car itself was worth.

It's generally going to be a less than ideal financial decision to lease, but it also won't collapse the used device market and it won't be that far off the practice of selling your old phone when you buy a new one.