It is important for animal advocates to be intellectually honest about the balance of the pros and cons to health from abstaining from animal products (being vegan). If they are not intellectually honest, their arguments will be discredited and discounted in the future by the general public.
Unfortunately, I think some of the popular medical doctors advocating for plant based diets, including Neal Barnard and Joel Fuhrman, have gotten a reputation for only presenting one side of the evidence. Similarly, I am not sure if the 2018 documentary Game Changers helped or damaged the arguments for plant-based diets since it was widely reviewed as insufficiently sober in its handling of a complex subject (https://www.menshealth.com/nutrition/a29067926/the-game-changers-movie-fact-check/). If I am wrong about this I would be happy to be corrected.
I have heard arguments recently that one of the most important components of healthy aging is minimizing sarcopenia, and this cannot be feasibly done without large doses of protein, including animal protein, spread out throughout the day (https://peterattiamd.com/donlayman/). These arguments are being advanced by respected academics, who are also arguing against school lunches going plant-based.
If animal advocates are going to suggest that the public abstain from consuming animal products, I believe they should address the sarcopenia point, and other similar points, directly and credibly. If this has already been done, I apologize, and would request that someone direct me to those materials. If it has not, I would like to suggest that this is a void that would be helpful to fill.
With regards to protein: studies consistently show getting enough protein to prevent sarcopenia or other diseases is not especially difficult, and in fact most people get substantially more than the recommended daily allowances. Also important is that consuming animal protein is positively correlated with all-cause mortality, while plant protein is negatively correlated. From what I can tell, though, this is probably because fiber is confounding the results.
Moving on from observational studies and to randomized controlled trials, it's unclear whether protein even has a meaningful effect on muscle strength, let alone something like lifespan, with some studies finding little or no effect from supplementation. It's really not clear protein would help with sarcopenia from the studies, but we'd honestly expect null results from theory alone. We know from surveys that the limiting factor in most people's muscle strength is physical activity, not protein. Most people get more than enough protein, and excess protein consumption turns into fat, not muscle. For most people, protein supplementation just can't help because they aren't moving enough.
The primary dietary determinants of health seem instead are mostly:
I'm busy, so I'll stick to looking at the evidence base we've acquired just on fiber supplementation. Here's a bunch of insanely overpowered observational studies with sample sizes stretching into the hundreds of thousands, all showing reductions in all-cause mortality. Meta-analyses of RCTs? We've got those in spades.
There is a reason every doctor's dietary advice is exactly the same: "Replace refined with whole grains, avoid saturated fats and replace them with unsaturated fats." The reason is because we have million-person studies showing these interventions reduce heart disease. Note that heart disease is by far the leading cause of death among the elderly; sarcopenia doesn't really kill, although it does make life substantially worse. And importantly, both of these are substantially more common in plants than in animal foods, which offers a pretty good explanation for why vegetarians seem to live longer, even after controlling for confounders. And also for why vegetarian diets seem to improve health markers in RCTs.
If you're wondering whether the above studies leave some room for including fish in your diet: My guess is yes, pescetarianism is likely to be slightly healthier, because of the aforementioned effects of polyunsaturated fats (including omega-3 fatty acids). This is why doctors also recommend the Mediterranean diet a lot, although it's not clear how much of the effect comes from fish and how much from vegetables. Omega-3 supplements definitely help.