[detaro]

One very simple point is: Forbidding third-party clients puts you at the mercy of the main developers caring about accessibility, a small-but-caring group of outsiders can't fix it. Whereas if you have an ecosystem of clients in an open system, a) users have a choice of options and can pick the one that suits their workflows best and b) don't have to rely on the main devs to care to be able to tweak one for their needs.

EDIT: this also supports the "bridge everything to IRC/Matrix/..." approach, since it lets people use whatever setup they already have working for their specific needs. And if Discord kills our bridge one day because "evil bot", well, rest of the group still works. And if something new and shiny comes up, sure, bridge it.