What do you expect the iPhone 19/20 to have that the iPhone 17 doesn't?
The tradeoffs will fundamentally be the same. You're not getting a > 100mm^2 sensor (with the glass lens required to image on it) on the iPhone anytime soon (or anytime ever really, unless the form factor of the iPhone dramatically changes).
Hopefully 48MP telephoto lens, with similar aperture to the main lens.
The telephoto sensor is already 48 MP on the 17 Pro, so that's a given.
I don't think we're getting a f/1.78 lens on the telephoto anytime soon. The widest aperture the tele's ever had was a f/2 on the 12 Pro, and that was back when the tele was a 2x (52mm equivalent, so not even really tele...). Maybe f/2.4 or f/2.2 (what the ultra wide has)? That'd already be a significant improvement.
The next big upgrades on the ultrawide/tele for the 19/20 generations will likely be larger sensors - they're currently 1/2.55", compared to 1/1.28" for the wide lens. I don't think they can fit three 1/1.28" sensors in there, so plausibly 1/1.7"? But that might mean that effective aperture remains constant.
(and yet none of that will beat an equivalent focal length lens on your XT3)