This would mean that low-performing students would need to make more than a year's worth of progress in a year, which might mean that districts would actually have to (gasp!) start providing services and intensive instruction to the kids that they identify as having learning disabilities.
Too often these discussions ignore the existence of low-IQ children. Treatable learning disabilities are hardly the sole cause of low performance, and I doubt they are the main ones. Many students are low-performing because they are low-IQ. Low-IQ children learn more slowly than children of average IQ. Expecting a group of low-performing students to learn faster than average students (not to mention high-performing ones) is unrealistic, although there will be individual cases of low-performing children (who may have had especially bad teachers or missed many school days in the prior year) who catch up.
