Nobody is actively discouraging that so you are just making up a strawman argument. That said cross training is very important for things like injury prevention. So if you just do something like running and nothing else at some point you are far more likely than not to get injured to the point that you have to completely stop or switch to something lower impact like cycling. And while cycling is great it doesn't have lots of the benefits of something like strength training so things like your bone health could still rapidly deteriorate as you age. So that means something relatively preventable like a fall leading to a broken hip that you never recover from can still happen to you. But yeah just do what you like and make sure you don't do anything you don't like in the moment

I did a cycling trip last year with people around that age and they were incredibly strong on the bike but were still relatively frail and had terrible mobility. I would never advise them to stop cycling because they were definitely way way healthier if they weren't doing that but it would really be nice for them to do some lifting for just one or two hours per week

Yes and people don't realize how much it helps with mobility. I used to not really train my legs and would just do lighter weight mobility exercises but I was still very stiff in some movements even after years. It was only when I added in proper weight training that my body started to loosen up. Tight muscles generally are weak muscles after all. Plus it's very rewarding seeing the weights gradually going up and actually seeing your muscles growing, not to mention the dozens of other benefits

I don’t hate weight lifting, but riding my bike is just glorious and fun and has a point (I go to the grocery store and buy groceries with my e-bike, then on multi hour rides up and down hills) so it’s something I actually want to do. Lifting weights feels good when I’m there but it’s a habit I have to build to go somewhere and do a thing. It’s just harder to integrate because it’s not literally just “go outside and hit the trail”.

I honestly don't know how people manage without it. Most of us sit all day for work. My body will literally demand that I hit the weight room after a few days, otherwise I'll have trouble sitting for hours, sleeping, etc.