What makes you think he doesn't have a plan?
'ALIEN: Covenant' clearly ends on the intention of following through on a plan; and simply heading into the back end of 'ALIEN' certainly isn't the End Goal - but that even would tie everything together into a cohesive goal. Scott is interested in going beyond that and a hard retcon or reboot isn't necessary. At all.
I never said 'parallel ALIEN Universe'. I said parallel narrative - one that co-exists with the established series not conflicting with it but rather enhancing it. We may even witness occasional dovetailing into to the older series. The discovery of why the derelict is on Acheron is a fine example.
What Scott says about 'ALIEN: Awakening' is subject to change, anyway. He may never even take the path to tie this series into 'ALIEN'. I personally think he will. But it doesn't necessarily have to be spelled out for your approval, either. He's already demonstrated he doesn't give a shit what some fans carry around as head canon for the past 30 years. None of that was ever concrete, anyway.
I'm really enjoying the ride Scott is taking us on. This is all heading into interesting territory. I want to see more. I want to see Colonial Security Forces engaged in combat with Aliens once again. I want to see the Engineers seek down David for some serious conflict. I want to see how Scott will tie the Derelict into this current series and thus cement the entire ALIEN Universe together. I also want to see what happens beyond those events in a series of movies that run parallel to the events of 'ALIENS' to 'ALIEN: Resurrection'. None of that is mutually exclusive right now. Importantly, it's going to take time and from the evidence we have seen won't be accomplished in a single movie.
Because his plan was over written by the studios decision not to continue with the Prometheus storyline ( which never contained Aliens).
Honestly I've never had a head Canon, I was quite happy not knowing. The one we got I feel doesn't enhance the originals or even compliment them all that well, although unlike some fans they don't and will never ruin my enjoyment of the originals, bar some slight thoughts about the Alien.
Having a big war/cross over with Engineers, David and Aliens doesn't seem to fit the timeline all that well, considering by the time we hit later movies in the Franchise they all imply that the Alien is all but extinct. Unless we do an AVPR and somehow have an intergalactic war without anybody noticing.
If they want to completely ignore all of that and we have some other way of going off into another series, then some clever writing could possibly do it.
I don't trust the studio though to handle any of this very well, they don't have a good track record and Scott seems to hold enough cards that they can't just be free of his version.