Preschool services are covered under Part B of the Individuals with Disabilities Education Act (IDEA). Which means that the child must be found eligible for services under one of the 13 eligibility categories. (In Arizona, there are 14 categories). Once they are found eligible, services, goals, programs and placement should be determined by the IEP team, of which the parent is a member, and should be based on the needs of the child. I would ask what they mean by "preschool services are limited." I would also look at what evaluations were done and if there are others that should be requested. Lisa has a really great resource on her site that I use often to determine which evaluations to ask for. It's called "IEP Evaluation Components" and is down toward the bottom of this page https://adayinourshoes.com/sample-letter-requesting-iep-evaluation/. (It's 3 pages long so make sure you arrow over to see all of it). Remember, you get data to determine needs. Needs determine services and placement.
To respond to your other statement about feeling alone in your decision to stop preschool services, this is really very common among friends I have with autistic children. I know many, many that decided to do ABA until the mandatory school age. My son was 6 when he received his Autism diagnosis and had already been in a full time school program at that point. We chose to do half days of ABA and school though for three years (K-2) and it was amazing the amount of progress my son made with the combination of both. You do what you think is best with the information you have.