I'd prefer 1x the size, 7.5x the weight and 3x the battery life.

You'd like your smartphone to weigh 1kg?

Edit: I assumed total phone weight there, not just battery weight. Apologies if you're just talking about the battery.

Unfortunately solid state batteries can't provide enough power for a smartphone. I would absolutely buy a phone that browsed slower for longer battery life + the ability to work below freezing though.

You can always connect a few cells in parallel and do whatever power output you want no?

If stuff is build for single cell li-ion at 3.7V, combining 2 cells at 2.4V is going to be a bit much.

I also believe cells in series are slightly more brittle, because of the potential for uneven discharge.

What if you put in both and charged the lithium battery with solid state one?

The solid state battery does not have enough power to meet average power demands. The phone would work for a short amount of time, then shut off until the small battery charged again.

What is the power density, then? It would have to be amazingly worse than a normal battery if it can't meet average phone demand.