Does the ADA require that every specific disability receive a tailored accommodation?
ADA requires that an individual assessment (a.k.a. "inquiry") be performed for each person with a disability.
For example, quoting from the Supreme Court's ruling on PGA Tours v. Martin:
As previously stated, the ADA was enacted to eliminate discrimination against “individuals” with disabilities, 42 U. S. C.§12101(b)(1), and to that end Title III of the Act requires without exception [emphasis added] that any “policies, practices, or procedures” of a public accommodation be reasonably modified for disabled “individuals” as necessary to afford access unless doing so would fundamentally alter what is offered, §12182(b)(2)(A)(ii). To comply with this command, an individualized inquiry must be made to determine whether a specific modification for a particular person’s disability would be reasonable under the circumstances as well as necessary for that person, and yet at the same time not work a fundamental alteration. [emphasis added]
In an obvious example, a wheelchair ramp does little to help someone who is blind. The person who is blind needs their own
disability-specific accommodation. Even this can vary for two people with identical disabilities.
A braille menu is an accommodation for someone who is blind but is useless to a blind person who cannot read braille. In this case, one person who is blind might request a braille menu while another might request that the server read the menu aloud. Both have the same disability and Disney is legally obligated to accommodate both.
From a practical perspective, individual assessments are unnecessary in most cases. Disney already has policies and modifications that will accommodate the vast majority of those with disabilities. DAS is one such accommodation. Presumably, there are thousands with Autism who visit WDW today, successfully using DAS to accommodate their disabilities.
However, since Autism is a spectrum disorder, DAS might not be enough in all cases.
Presumably, no one asked for an individual assessment when GAC was in place because GAC was enough for all, and probably more than enough for most.
Within the confines of ADA, Disney is required to provide only "enough", not "more than enough".
If an individual with a disability thinks that what Disney offers as part of its standardized modifications is not "enough" to accommodate their specific disability, they can compel Disney to perform an individual assessment to determine what is "enough".
The important point to keep in mind is that Autism is a complex disability with widely varying degrees of symptoms.
In the recent lawsuit, Disney argued that they performed an individualized assessment of the plaintiff and, based on that assessment, determined that DAS plus 24 extra readmission passes (for a party of 6) was "enough" to accommodate their specific disability.
Fortunately, another cast member overheard this conversation and took over. 
