I think any new expansion bought should include all previous expansions, for no additional cost. That would be the easiest way to maintain the game only 2 versions to update. One version is base Elite Dangerous, and the other is Base game and all expansions. A lot cheaper to maintain 2 versions then 3, then 4 then 5 etc....
Of course I can hear some cry, it is not fair I paid for Horizons and now you give it away for free. Of course it isn't free they had to buy the latest expansion to get it, but you get the point some will moan and cry over it, if they go this route. I was in the alpha and have life time expansion pass. So to me it is free, but I think any expansion going forward should include all the previous ones in the price.
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Calebe