So I've been giving this question some thought recently for a story idea I'm working on.
(Writers who may be lurking, be forewarned: story ideas ahead.)
What if the Delphic Expanse was extremely wide (maybe not 2000 ly, but wide nonetheless), rather tall, but not very deep (maybe 5, 10 light years)? A bit like a pizza cutter cutting into the Milky Way. It would be a nearly impenetrable wall. Extremely influential in local astrography, but not necessarily overwhelming in the number of planets contained within.
Assuming it was about 50 ly from Earth in the opposite direction from Qo'noS, it could explain why Earth et al was never overrun by the Cardassians, the Tholians, the Tzenkethi, or the Breen: they couldn't get past the Expanse (and had no incentive to do so).
Indeed, if the Expanse was frightening enough, it could've encouraged the Romulans and Klingons to expand in the other direction long ago, again, leaving Earth et al free to develop.
it would explain why it took so long for the Cardassians et al to encounter the Federation (and vice-versa); even after its destruction, I bet all the Local powers would've given the former Expanse a wide berth, choosing to focus their efforts elsewhere.
In some ways, the existence of the Expanse for a millenium between ca. 1150 and 2154 explains something that's bothered me for a while now: why did these great empires (Klingons, Romulans, Cardassians, Breen, whom we know have existed for centuries before the Federation developed) conquer Earth before we had even invented electricity? The Expanse suggests an explanation.