I hate to keep beating a dead horse, but I really must disagree with your opinion, OP.
Games have been many things for a while, but they're becoming less, not more.
The first question game developers should ask themselves is not "How do I fit this into a shooter", but rather "How do I make something truly unique?"
When I first picked up a controller, it was to play Super Mario Brothers. Over the next 15 years, I owned nothing but Nintendo consoles. This means I played platformers, action/adventure games, and other titles that were accessible and enjoyable. These games were traditional, and "games" in every sense of the word. I think you'll agree that I did a lot of running, jumping, sword-slashing, and occasionally shooting. Everyone remembers with rose-colored glasses the games of their childhood, and these Nintendo games meant a lot to me.
When I became an adult, I learned about this genre called the "adventure", something which had eluded me my whole life. Apparently people like Tim Schafer had been entertaining people with characters and puzzles, but no jumping. No running. No shooting. How could this be? You're just...thinking, and not in the Tetris puzzle-solving sense. Your enjoyment comes entirely from a set of scripted events that you enact by putting a thing in the right place. What's more is that you usually can't die in these games. This may be a gross oversimplification of adventure games, but it will do for now.
So I learned about different genres. So what right? Well, as I was talking to people raised on the PC I also learned about simulations. There were games in which your goal was to create something, and maintain it over a period of time. Perhaps a city, ship, rollercoaster, or farm would fall under your control. Again, in many of these games, you don't lose. You only adapt.
Finally, before I knew the dreaded word "non-game", I played a 2002 Gamecube game called Animal Crossing. This is a game in which you make friends and similarly carry out tasks that are centered around communication. These tasks include running errands, sending letters, and simple commerce. The interesting thing about Animal Crossing is that not only is there no way to lose, but there is no challenge of any kind. The game cannot get harder. Despite this fact, it won the AIAS (a big deal at this time) RPG of the Year award for the year of its release.
Now I bring up these examples to show you your concerns are nothing new. If the enjoyment derived from having no challenge is invalid, then "gameplay" needs to be reevaluated.
Games have evolved constantly over the years.
In the 1980s through the mid 90s, many games involved a character moving from left to right, often equipped with the powers of jump and attack.
In the late 90s, we started seeing many of these same gameplay mechanics adapted to the 3D space.
Nowadays, gameplay is a mixture of many ideas that are the result of what the market has indicated it wants.
The word "gameplay", as defined by the thread author only serves to further constrict genre variety.
Journey is a game because not every game needs an adversary or challenge.
