It's probably possible to replace the nutrients without eating meat or fish.

As far as the environment goes, it seems we could go a long way if people cut back significantly on how much meat they eat, or if (as our ancestors did) we do better about fully using the meat products we do consume. I have been amazed at how much mileage I can get out of one chicken, winding up with highly nutritious bone broth, if I put my mind to it.

I describe myself as. vEGGan. I consume one to two eggs a day in various meals. I also take a high quality collagen supplement .(yes I do understand that this is heretical to some vegans...it is derived from bones and cartilage of animals and fish).

I highly recommend it. I am over 65 and take no presentation drugs and walk 3miles a day at a fast rate.
It's not for everyone, of course, but it works for me.

difficult for most sans any and all forms of fish/animal-based supplementation. Add to that the need for adequate vitamin D (which you apparently supplement naturally with outdoor physical activity) and weight-bearing movement--all of which you seem to work to make sure you maintain.

I'm absolutely in favor of eating more plant-based foods. Strict absolute vegan diets, considerably less so for many/most people. And yes, of course not smoking and good genes make a difference as well, but diet is incredibly important.

I become a vegetarian in 1976 because of my concern about animal cruelty and factory farming.
There are so many more options available for Vegetarians today. Ultimately now it is about a sustainable diet that does not destroy the planet.

Diet For A Small Planet, by Frances Moore Lappe
Published in 1971, was the first major book to note
The environmental impact of meat production
As wasteful and a contributor to global food scarcity.

Even if you have no interest in vegetarianism,
This is still a great read.

...like smoking (big cause of osteoporosis), heredity or hormone use. I'm vegetarian but still eat dairy and eggs. My last bone scan was very good.

Our teeth show that we are designed as an omnivore.

Feel like they buried the lede here; from the study:

Characteristics of the 26,318 cohort participants at recruitment are summarised by diet group in Table 1. Over a median follow-up time of 22.3 years, 822 hip fracture cases were observed (556,331 person-years), corresponding to 3.1% of the cohort.

So that's a 33% higher risk against a 3.1% average. Vegetarians have a 4.1% risk of hip fracture compared to the average of 3.1%, with an absolute delta of 1%~.

Meanwhile, also in the study for some reason:

On average, at recruitment, pescatarians and vegetarians were younger than regular meat-eaters, reported higher education levels, were more likely to have professional or managerial jobs and less likely to have routine or manual jobs, and were less likely to be married or have any children. BMI was lower in vegetarians (mean (standard deviation, SD) 23.3 (3.9 kg/m2)) and pescatarians (23.3 (3.5 kg/m2)) than in regular meat-eaters (25.2 (4.4 kg/m2)). Prevalence of CVD, cancer, or diabetes at recruitment was highest in regular meat-eaters (n = 1250 (10.2%)), and lowest in vegetarians (222 (5.8%)).