I doubt they will abandon Elite before implementing at least one more major expansion.
1. They are building a new lighting system in Q4. Imho, this would be a wasted effort unless it was necessary work to implement atmospheric planets, or volcanic ones for that matter. Current lighting would be totally fine for current planets and environments in ED.
2. They are building a new scatter rock system in Q4. This too, would be a good feature to build upon when we get trees and other objects in planets. Again, I don't feel this would be necessary for gameplay reasons.
3. Implementing atmospheric planets in a REALLY basic form isn't that hard. It would be foolish not to include this as it would bring the game to another level visually. The reason why it's not implemented yet is because they plan to cash in on it in form of another major expansion.
4. They have many of the features necessary for atmos already implemented for PC and JWE. Why not use it?
5. Lifetime expansion pass doesn't really make sense unless there's at least two paid expansions. I guess FDev isn't obligated to do more than one, but I have faith in that they honor their promise for lep owners.
6. Playerbase is still active, and there is a large target audience willing to purchase the next expansion.
I think it's a smart move on FDev's part to improve core mechanics with Beyond while working on the next expansion. Personally, I feel they've missed their mark on making the game fun. It would help to expand their customer base if they manage to reduce grindiness, improve on fun and fix major issues in game prior to releasing the next expansion. Beyond seems to be doing exactly that, although it remains to be seen if FDev succeeds in this.
Another reason why we haven't yet gotten news about the next expansion from FDev is that they have a habit of announcing them relatively close to being released; Horizons was released some 4 months(?) from initial announcement. And it seems FDev had their hands full on development of 2.1, 2.2, 2.3 and 2.4, so they haven't had time to begin work on 3.0 or 4.0 in full until now. It may also well be that the season model in Horizons was a mistake; people weren't ready to pay full price upfront for an incomplete Horizons.
Personally, I think they'll announce the next paid expansion in Q3 or Q4, but scope may vary depending on how well Beyond is received by players, how many copies FDev expects to sell, and how much development power is taken by other projects like JWE.