A zero with a decimal and infinite 9’s after it would not be one. However, the “…” implies we are looking for what the sequence is APPROACHING as you keep adding more 9’s after the decimal point, not what the sequence is. The sequence would constantly approach the value of 1 without ever being one. But we are looking for what the sequence is APPROACHING. Thus, since a zero with a decimal and constant 9’s after it is always approaching 1 (even though it never becomes 1,) we define 0.999… as being equal to 1.