In terms of the number of countries and total viewership of the world circuit games, Sevens probably is more popular than XV's.
The fact that it is an Olympic sport now, has raised the profile (and standard) of Sevens in many countries, and this will hopefully have a flow on effect to XV's.
I do watch both, and find that their are those that insist that more tries = a better game in XV's (I don't agree with this), yet a lot don't watch Sevens, which is naturally geared to the scoring of more tries, due to less players/more space on the field.
I can't see any reason for changing the Seven minute halves (10 minute halves in the finals either), the fast pace of the game, and the number of games played over a two (sometimes three) day period, means it's taxing enough on the players.
The two games are already different from one another in enough ways to see specialist evolve due to the different physical requirements of both versions of the game, and I can't see one supplanting the other, nor World Rugby promoting one more heavily, to the detriment of the other, as they are both great tools for expanding the game globally, and both should also provide good revenue streams at the top level.