I've been vegan for about 2 years and vegetarian for the 18 years before that. I transitioned to veganism over a period of about 6 months by cutting down or eliminating things one at a time, and researching how best to replace those items nutritionally and in my cooking. I don't think going vegan over night is the best idea if you want to stick with it long term, but I think Veganuary is a great time for people who were already thinking about going vegan to try it out.
I think most people do rely more on replacement products when they first go vegan, and then start experimenting more with whole food meals as their cravings for animal products lessen. If it's your first time trying to eat vegan then it's much easier to just replace your meat sausages with soya sausages and call it a day, than to suddenly learn how to make lasagne with lentils or whatever. And veganuary makes this easier because these fake meat products are often massively reduced in price. So the benefit to the person is that it gives them an easy way "in" and helps them see if veganism could fit into their lifestyle in a sustainable way.
It's also a more socially acceptable way of trying it out, because you can just dismiss questions about your new diet as "oh I'm just trying out veganuary" rather than saying you're actually becoming vegan which often comes with a huge amount of pushback and negativity from non vegan family and friends.
Some people will stick with it, but I imagine the most common scenario is that people end the month having found one or two products they really like (I know loads of people who now prefer soya mince to meat mince, or plant milk to dairy milk) and just incorporate these into their diet as a way of slightly reducing their meat consumption. And tbh I also consider that a win. Even if some people just have 1 fewer meat and dairy based meals a week, that's still an improvement from an animal welfare POV. Depending on which products they're choosing it may also be an improvement from an environmental POV as well.
Most vegans I know don't target their activism on a vision of the world where everyone is 100% vegan as most of us know this isn't realistic, but every step that people take to reducing their consumption of animal products is a positive step in my book. If veganuary leads a few people to choose veganism and a few more to start doing regular meat free Mondays or whatever, then it's done some good.
Speaking for myself I bloody love all the fake meat products and use veganuary to pack my freezer with all my favourite junk food that I can't normally afford 😋