You can still doubt that quantum computers are physically realizable. Perhaps the number of qubits needed for error correction will turn out to increase fundamentally faster than the number of useful qubits, or perhaps there is some other fundamental limit that we will hit.

But the maths is clear: if it is at all possible to create a quantum computer, it will be faster for certain specific problems than a classical computer (in particular, it will be faster than a classical computer at simulating quantum phenomena).