The issue is that you do NEEd air in order to use the props. So there is a limit to how high you can go.

Air resistance vs not having enough air to push.

Basically - you can design the prop for high altitude, but then takeoff is gonna not be great. You could design a variable prop system (or even have 2 different ones?) but then weight / complexity.

https://www.hindawi.com/journals/ijae/2018/5782017/

If you go supersonic you get enough air in even when flying higher.

His plane is VTOL so the overpowered engines for supersonic will work well for liftoff.

Getting higher is no easy feat, especially given the energy density of the current state of the art batteries.

Not to mention additional complexities going above 10,000 ft that adds weight: pressurised cabin (or oxygen) and anti-ice and de-ice systems and heating.

Exceeding 10,000 ft. is orthogonal to needing anti-/de-ice systems. In fact, you're often less likely to need them at the higher altitudes because it is too cold for airframe icing to occur. Counterintuitively, if it is too cold, you no longer have icing problems.

You need to pass all of the feet leading up to 10'000 feet though.

You can use the batteries as structure and they produce heat that you need to cool anyway. Seems like there is a lot you can do with that once you start optimizing everything around the battery.

I don't know how much weight a anti-ice/de-ice could add.

My point was only that once you think about it and redesign everything around the new constraints, you can come up with things that we don't yet think about.