SPOILER: Show
How does that make it impossible? What assumption is there to have that David killed off ALL the Engineers? For a species that we see has a history of sending their people off across the galaxy seeding other planets it seems presumptuous to assume ALL the Engineers are in that one city at that exact moment. For a movie that heavily features colonists everyone immediately assumes that one city on that one planet was it. That was ALL the Engineers. For all we know that too was just a colony. Either way, in an interview with John Logan - who has/is writing a draft of the sequel - for BMD/Drafthouse's special Alien issue he was asked if we'd see any Engineers in the next entry (if it happens) & his response was "Well, (Answering) that would be giving you a clue for what I'm writing now." hinting at the return of the Engineers since it'd be far simpler to say "Nah, they're done". Seems silly to jump to the conclusion they're all dead. A lone Engineer (or even a ship of Engineers) out there in space comes home to a massacred city of their people & then pursues the Covenant/David & in the final act an Engineer steals some eggs, gets Facehugged & crashes on LV-426 seems like a very natural progression of the story Scott is telling over these (possibly) three films. Just like how mad eugenicist David creating the Alien is a logical progression from Prometheus.
Nothing is IMPOSSIBLE.
Nothing is IMPOSSIBLE.
And the mural is just a goof that comes along with telling sequels over long distances of time. Things change in the creative process that can't be foreseen. Nothing is set in stone. Characters get recast (The Hulk in Marvel), imaginary worlds undergo drastic revisions (Alfonso Cuaron basically rebuild everything about Harry Potter when he took over & nerdrage was nigh) & stories evolve. It would be hypocritical to attack Scott's two prequels for a mural seen in one shot when Rebuild (I really have to write an essay about how similar these two new-sagas INCLUDING how little they care about superfluous side characters & super strict continuity, things both are routinely attached for) spends two movies actively building up Eva Mark 6 as this ALL-IMPORTANT thing & then just throws it aside almost entirely. Not necessarily because Anno was pulling a bait & switch but because he realized the story he wanted to tell didn't need Mark 6, The vision the filmmakers had evolved from one film to the next because the films in both franchises are not being made by a committee of TV minded producers who are trained in episodic stories. They're being made by artists who have their own weird/wonderful interests & fetishes. You either roll with the punches & go with the story or you nitpick about how thing X in this film doesn't correspond to something that occurs three films down the line. Like how unimaginative Aliens bros are saying Scott has erased the Queen Alien which is nonsense, a key part in the creation of life would be giving said life the option to procreate, hence David creates a Queen at some point... or after settling on Origae-6 things get out of David's control - continuing the circle these films have shown where the life you create destroys you, the Engineers are killed by their biological weapons/humans, humans are killed by the AI they create & the AI is destroyed by the PERFECT ORGANISM (to quote another AI in the original Alien, makes sense a robot would love something created by a robot).
I think Scott was telling the film he was focused on that moment. When/if it comes time to tie things into the original Alien it wouldn't take much to do so.