I would expect long-term married people to place more value on their ability to grow as individuals, in tandem with their partner, who does the same. Otherwsise, doesn't it become stifling to do everything in unison?
After 27 years, I would imagine most couples have settled in for the long-haul and are content to just enjoy each others' company, their families and their hobbies. How would a couple "grow together" after all that time? Let's say a couple took a cooking course together and mastered french cuisine, and then opened a french restaurant to local acclaim. I guess that could qualify as "growing as a couple".
I haven't paid much attention to the topic, but I think I just assume that by age 65, married long-haulers stay married because they are used to each other, mostly fond of each other, and have learned to adapt to each other's idiosyncratic habits. At age 65, wouldn't they be more interested in seeking ways to express their individuality rather than coupleness? (We have several long-married readers, maybe they understand and can explain what growing as a couple means after 25 or more years).
Perhaps the shorter version is that the Gates have different projects they want to spend their remaining time and their fortune on, and they are very cognizant that this next phase of their lives could be their last productive one. (Even billionaires don't have the ability to see into the future and know whether they will be alive in 15 years and even if they are, whether they will have all their faculties.)
In the past, they had time to jointly focus on one of their pet projects and then jointly move to the other's priorty project. Many philanthropy projects take years to see fruits. Perhaps the Gates are simply realizing their time on this earth is dwindling, the projects they are mutually interested in have been established don't need their continued participation and they don't want to spend whatever productive time they have left on projects they are not individually invested in.
Or maybe, it's as simple as one of them fell in love with someone else.
Whatever the reason for their split, I think it's sad and I wish them well. As for respecting their desire for privacy, I think they forfeited that when they each announced on Twitter they are getting a divorce. If they truly wanted privacy, why not just hold the news until TMZ stumbles across the court filing and then respond "No comment"?