I know lifelong smokers. The human body is resilient. If your argument is that veganism is healthy, you need a lot more than "I'm vegan and I'm not dead".

I mentioned elsewhere about protein intakes. It's not a controversial take that protein is one of the most important things we need in a day, that protein is easiest to find in meat, and that our body isn't as good at digesting plant protein. For the rest, telling someone to go plant-based when you need a lot more than just a multivitamin to hit the Iron and B-12 content you need.

Whether or not veganism can be healthy (it might be), it is a known quantity that naive veganism is absolutely unhealthy. So my problem with "getting them vegan is easier than getting them to cut 10% meat" is that you're trying to create naive vegans. That means you're trying to create smokers.