While some businesses may not be ADA-compliant, that does not make it right.
Where you want to look to is the ADA guidelines. The ADA put out a user-friendly small business guide available at the link below.
The ADA recognizes what burden modifying an older structure may impose on a business owner:
"Therefore, the ADA requires that accessibility be improved without taking on excessive expenses that could harm the business...If you own or operate a business that serves the public you must
remove physical “barriers” that are “readily achievable,” which means easily accomplishable without
much difficulty or expense. The “readily achievable” requirement is based on the size and resources of the business. "