Depends on what your definition of ‘happy marriage’ is.
Is it not being divorced, raising kids successfully, having companionship, still having sex etc.
After the end of a 20 year marriage I’m of the view (similar to
@SW15) that the sheer passage of time dooms most LTRs (there will always be outliers).
Many factors at play - becoming unattractive or fatter over time, people growing apart or changing compared to when they first met, loss of frame, hedonic adaptation, complacency, selfishness, the stress of raising a family, the monotony, irritations and familiarity of living together and having sex with the same person (most people crave variety) means that most marriages eventually end up in a state of comfortable boredom where desire and passion have all but disappeared.
Esther Perel talks about the need to balance intimacy (companionship) and desire (lust/sex) in relationships as time goes on and that the factors that promote desire (distance, mystery, unpredictability, insecurity etc.) are the polar opposite of the factors that promote intimacy. Most LTRs, if they go for significant periods of time, end up in a situation where intimacy is all that is left.
You can limit the rate of decline (e.g. having a younger/very attractive partner, not living together or having kids, being open to changing things up in the bedroom, putting in the effort to maintain your fitness and appearance, showing appreciation, doing different things together etc.) but most LTRs will end up in the same place eventually.
I think the best you can do is enjoy the ride for as long as it lasts (the divorce rate is 50% for a reason) and be prepared so it’s not a shock when/if it happens.
I’m certainly not advocating never getting married and never having kids (having kids for me was the best thing I have ever done despite the end of my marriage) but you need to go in with your eyes wide open.
If you don’t intend to have kids then I really question why you would ever want to get married or live together purely from a desire maintenance and wealth protection perspective.
Better to live by yourself and be a serial monogamist or spin plates and enjoy your freedom, peace and quiet (qualities you value as you get older, your T levels decline and you have experienced a LTR of many years’ duration).
At my age the price of ‘in house *****’ (which is a rapidly deteriorating asset if close to my age range or will come with a heavy dose of negotiated desire if a younger model seeking a provider) is no longer worth it.