If you promote the local feed, your posts get attention, which means user engagement. So, user engagement would have a trend to stay in its own instance, which results in bubbles and is certainly not good for discussion quality.
I think this is too much of a generalization. Certain discussions even benefit from a certain amount of shielding from the outside world. I think there are mechanics working both ways, and to generally equate local feeds with reduced discussion quality is a poor argument.
Also, how would the addition of another feed (read: the selectable option for another feed) change anything about that? Instance owners who “push users” to their Local feed (as you unecessarily dramatize it) could still choose Local as their default, even if you requested new feed was implemented.
In both scenarios (with and without the new feed), users can freely select another feed anytime (because no one actually pushes them), or even define one permanently in their settings, overriding whatever default the instance owner had selected.
The new feed would do nothing about the situation but give instance owners another option to “push users”, and users another option to select from.