What if Ridley simply doesn't want the biomech features on the alien anymore? I think the alien in next movies will look the same as they did in Covenant. He probably doesn't even remember what the original alien looked like.
Then he should probably rewatch the first film
If a director wants to maintain good continuity they should look at the previous films as its a good way to ensure no contradictions happen, however, Ridley does not seem to care for continuity it seems which makes me sad as I hold him in higher regard than the likes of Fox. I didn't think he would go down this path.