"Because of the Turing completeness theory, everything one Turing-complete language can do can theoretically be done by another Turing-complete language, but at a different cost. You can do everything in assembler, but no one wants to program in assembler anymore"

The Turing completeness theory states that any computation that can be done by one Turing-complete language can in theory be done by another Turing-complete language. This implies that the very same job can be accomplished in different languages, however the cost of doing so may vary. Yukihiro Matsumoto is suggesting that although it is possible to do whatever in assembler, it is not a desirable language to program in any longer. This is likely due to the truth that it is a low-level language, suggesting that it requires more effort to compose code in it than in higher-level languages. Additionally, it is not as efficient as other languages, so it may take longer to finish a task. For that reason, it is more helpful to use a higher-level language that is more efficient and simpler to use.