Why on earth should it be 'promoted' as a route to happiness though? It's a valid lifestyle choice increasingly embraced by many, but it's no more likely to guarantee you happiness than having a family might. In fact loneliness is known to be a killer, especially of men. If you lose your life partner then you are far more likely to end up lonely, depressed and isolated in later life, with no children to feel connected to, or to want to carry on for.
All being child-free guarantees you is more free time and more money in your pocket, assuming you earn reasonably well to start with and children are not the cash cows that unlock access to better housing and more benefits for you. Having a life with no children doesn't necessarily guarantee you less stress, or better health or a better career or a more enriching or interesting life. I'd urge anyone who isn't going to have children (whether through choice or otherwise) to seek an enriching and interesting life and to avoid falling into a hamster wheel of tedious responsibility. I'd urge them to harness the freedoms of childlessness and enjoy them to the fullest, but ultimately you still have to make that life happen for yourself. It's not a guarantee that comes with being child-free. When I look around at most of the child-free (by choice) couples I know, their lives are as ordinary as everyone else's, which I don't really understand. It seems like a wasted opportunity to me.
There is less pressure on you, so if your life is going to shit and your job is giving you burnout you can jack it all in and downsize, go off round the world, live off-grid in a yurt or whatever, and not have to think about the needs and wants of anyone but yourself. You can devote yourself to hobbies and passions and travel, and if a relationship is bad you can leave it without fear of how it will affect your children's security and happiness. You don't need to 'settle' for a sub-par relationship, or tolerate poor behaviour for fear that you don't have time to find anyone better to have children with. For women in particular, that must be incredibly empowering.
There are undoubtedly many benefits to being child-free on paper, but very few people choose not have kids based on a tick-box exercise of rationalisation. If you feel an unexplainable yearning to have them then no amount of rationalising or 'promoting' childlessness is going to make a difference. No amount of extra money in the bank or extra 'me time' is going to fill that void in your life. And what, ultimately, is the point of amassing a load of money anyway, if you don't have children to pass it on to? It's the very thing that drives us, and has always driven us, whether it's to go out hunting and gathering or to push for the next promotion.
DH and I often ask ourselves how much money we'd be worth by now, if we hadn't had three kids to shell out for. DH has been a high earner so it would be an eye watering amount frankly, assuming he'd made the same career choices and I had carried on working instead of being a SAHM for years and years. But what on earth would we have done with it all? How many fabulous holidays and nice cars and expensive restaurant meals can you have, how much golf can you play or gig/theatre going can you do before it all just feels a bit like a giant displacement activity to cover for something else that's missing?
Someone will only find true happiness in childlessness if they've chosen it. And in most cases they choose it because they come to realise they just don't have any strong maternal/paternal insinct and the idea of having children actually makes them panic and recoil in some way. I can't imagine many people make a conscious decision to ignore that biological urge or deny themselves the permission to give in to it, in favour of a simpler, easier life with fewer pressures and more disposable income. That's rational and there is nothing rational about wanting a baby. It's a instinctive urge that most of us can't switch off.
I totally get that people who are childless not through choice, could learn to be happier by embracing all the positives instead of dwelling on the negatives. They should absolutely grasp the opportunity to fill their lives with every enriching life experience possible, that most of us with children never get the time and space to do. Because if a life without children is your unchosen destiny then you may as well make that destiny as fulfilling and thrilling and distracting as possible, instead of living a life of dull routine and responsibility and always feeling sad and unfulfilled.
But frankly you are only ever going to sound like a patronising knob if you tell them this, especially as a person with children.