Can I reach band 7 in IELTS Speaking with simple sentences?

Not on their own. The band 7 row requires a range of structures used flexibly, error-free sentences that are frequent, and both simple and complex sentences used effectively, so a performance built only from short forms cannot satisfy it. Simple sentences still have a job, because they carry detail and keep the answer moving, but they have to sit next to complex structures that fit the question. Because a few basic errors are allowed at band 7, the aim is not error-free speech but structures that are varied, appropriate and complete.

There is one legitimate exception: a performance can still sit at band 7 when the simple sentences carry the content and the complex ones appear where the task needs them, for example a comparison in Part 3 or a short sequence of events in Part 2. What will not work is cramming every answer with a memorised complex shape, because the examiner hears the same structure used for a question it does not fit, and the flexibility wording is judged on whether the structure matched the job it was doing.

IELTS Speaking Band Descriptors: What Grammatical Range Actually Costs You →