[gji]

Disclaimer: I used to work on trapped-ion systems.

I hear this a lot about trapped-ion qubits, but in their current state, no technology is scalable. The usual suspects are superconducting, quantum dots, or diamond NV-based qubits, but each technology has their own scalability problems. Superconducting qubits suffer from either requiring massive (vacuum tube sized) cavities or a ton of crosstalk. Good luck isolating superconducting LC circuits from one another on one circuit board - a reason why these are limited to only a few qubits. Quantum dots have pretty awful decoherence issues and I'm not sure they can be implanted deterministically. Moreover, I don't think people have demonstrated non-photon mediated entanglement, which is not particularly scalable. NVs have the same implantation and entanglement problems, though at least they can be used at room-temperature. Superconducting and quantum dot technologies require million-dollar dilution refrigerators and large amounts of (expensive!) helium-3.

Obviously, these technologies also have their advantages over trapped-ion qubits. But trapped-ion proponents also have their own roadmaps to scalability (surface traps created using lithiography).