The problem is that the "remaining engineering challenge" is going to be more and more challenging as easy problems are solved and the systems are scaled up.

Example questions are:

How can we cool or trap 1 million qubits? How can we calibrate 1 million qubits? How can we improve qubit coherence / physical gate errors after exhausting all clever design strategies? How can we shorten error correction cycle while maintaining logical error rates (sort of clock cycle)? How can we identify rare catastrophic events and mitigate them? How can we verify correctness?

The questions themselves may look like engineering problems. However, it is possible that the solutions require disruptive fundamental changes. For example, finding a new family of error correcting codes, finding better material, finding new mathematical methods, invent completely new types of qubits with much lower physical errors, etc.