I would accept that they're allowed to make their own decisions about what they want to enjoy, and while I might recommend things a bit outside their usual comfort zone from time to time, I wouldn't lecture them about it or insist they are doing themselves harm.
To each their own, but I personally don't think it's healthy, and would border on an obsession possibly. So I would lecture them.
My grandfather told me a long time ago, "don't be nice, be kind". It took me a long time to understand what he meant by that. But being nice, means you want people to like you. So, for them to keep liking you, you don't want to tell them things they don't want to hear. You don't challenge or push them to become stronger. On the other hand, being kind means helping people even if they don't like it. You kick them in the butt when they are doing something harmful.
My analogy to the food was that eating only one food type is literally not healthy for your body. Only being into one genre or worse, one specific instance of a genre is I believe also unhealthy. Smoking is someone's choice, as is overindulgence of eating sugary foods. That doesn't mean it is healthy for the person.
Am I suggesting that it could be as serious as smoking or overeating? Yes, I do. If you wish to call that narrow-minded, that's your choice. But I would literally try to get someone out of an unhealthy habit like that and not just say "well, it's your life". I can't force them to of course, but I would definitely explain the dangers and be nagging about it (until I realize it is hopeless). Exposure to only one way of thinking or doing things leads to a closed way of thinking and understanding things. It is easy to measure physical health, as my example with smoking or eating too many carbs and becoming diabetic. But this same over exposure to the mind can be damaging just like it is to the body.
If the argument is "it's just a game", I would argue that gaming is also art. And art is what leads us to appreciate things and see both beauty and ugliness. If all we ever see is pictures of one ethnic group with a certain body type, then we may formulate in our minds: "this is what beautiful is, but I don't look like that". What about only seeing the world by playing only one genre with one game system? Would that not also restrict our way of thinking and seeing things? Gaming, for many people, is also their safe (and perhaps only) outlet for expression, and to interact with and experience another world. In other words, it can be a therapy. So I dismiss any notion that roleplaying "is
just a game".
What I believe IS narrow minded, is saying "you must only like this genre" or "there's something wrong with anyone who likes that game system". And that's not what I am saying. It's perfectly fine to be exposed to many game systems or genres and realizing you really only like one. At least that person will have been introduced to other settings and systems so they have the groundwork and basis to compare things. And even then, I'd recommend to occasionally try out something new in a genre they previously dismissed. It's always possible whatever system they picked out for a certain genre just wasn't that good. One of my exGF said she hated Science Fiction, but I told her to try watching the rebooted Battlestar Galactica. After much protesting, she finally watched it, and loved it.
It runs whatever I need well, which means that it struggles to be a technical, reference/simulator or OSR (the table-heavy aspect). Those options are possible, but without another modder's efforts, I'd have to write up those modules for it myself. Or pick up the books in question, and write an easy conversion rules-module.
