If developing a skill, even shakily, means someone will lose their accommodations, that makes it impossible for someone to use that skill. That’s a problem.
For instance, people who can walk often still need wheelchairs. Denying wheelchairs (or wheelchair accessible transportation, or other accessible things) to people who can walk doesn’t make a wheelchair any less necessary. It just prevents people who can walk a little from ever doing so in public. (And prevents some people from developing that ability at all.)
Similarly, a lot of AAC users can talk, or can talk some of the time. For many people, the best form of communication is a mixture of speech and a communication device. Often, when people speak, they are treated as though they are faking their need for AAC. Or that they’d be able to use speech as their sole means of communication if they tried harder. That doesn’t make AAC any less necessary. It doesn’t make speech any more possible. All it does is make it impossible for someone to use both speech and AAC, which deprives them of communication options they’d otherwise benefit from.
Similarly, a lot of people who can read visually also need screen readers from time to time. If seeing someone read standard print means that you won’t let them use electronic formats anymore, that doesn’t give them new abilities. All it does is stop them from reading.
Wheelchair users have the right to do what they want with their legs and AAC users have the right to do what they want with their voices. People have the right to read in a combination of ways that are possible for them. There are numerous other examples. Those rights matter, and they’re often ignored.
tl;dr People with disabilities who use equipment or adaptive strategies are often prevented from doing things in the standard ways too. They’re expected to either do things in the approved disabled way or the approved normal way. This is wrong. People should be able to do things in the way that works best for them. (Which is often a mixture of different ways, some of which are used by nondisabled people as well.)