If i remember correctly, when Kaplan leaved the team, there was talks that Keller isn’t interested in PvE, so that’s probably the answer. He, and probably many other lead devs simply aren’t interested in it.
Invasion and Wrath of the Bride were leftovers from previous development, and Cosmic Crisis and Trials of Sanctuary probably was made by small team on their own initiative, because they wanted it. (and to promote new skins)
We also have the 3 released missions
We have more, actually. 3 Archives misisons that simply needs to be updated (i’m not buying their bs on hard work it will take, you have the tools, that’s what you’ve spent 3+ years on), Junkerstein’s Revenge (which they did updated, interesting), Wrath of the Bride, Cosmic Crisis (remove imposter), Underworld, 3 Invasion missions. (Trials and Hero Gauntlet would require rework, still could be added in some form).
That’s already 10(12) missions, add All Heroes mode, weekly rotation with different modifiers (which you already have and used before) and add a free lootbox upon their weekly completion (like in Starcraft II Co-op).
Stadium isn’t necessary, and imho, won’t work with those missions, only if you’ll be able to fully assemble your build before the mission, and cash / powers you have depends on the difficulty (to stimulate you playing on higher difficulties).
Oh, you must be able to complete daily / weekly challenges in them as well, probably with slower progression and required victory.
TL;DR: Yep, they can make PvE at any time, they just don’t want it (also they 100% would wanted it to be competetive)