Both of these horses had been retired for at least 10 years - so they'd stopped 'making money' years ago, and I can't imagine horses like that have been cheap to keep in the last 10 years, especially with health problems needing to be treated.
My horse of a similar stamp was 20 when I had him PTS, he had arthritis and Cushing's which brought with it increasing bouts of laminitis, he'd been retired since 15 and living out because of the arthritis but that management became counter productive with the management that recurring laminitis needed - plus that management made him utterly miserable (and if you don't know what those conditions are, or how they interact/need managing in horses then you're in no position to comment).
He'd had a very easy life compared to an Olympic dressage horse, and at 15 the vet told me he wouldn't see 20, and was on borrowed time from when the Cushing's advanced and stopped responding to treatment.
And he lost one of his field companions 6 months before, he had other companions but he was bonded with my friends mare - I honestly wish I'd let him go when she went, because he missed her, stood as close as he could get to the spot she was pts on daily and joined her 6 months later anyway, I could have saved him that.
While I agree that with some competitors and horses there is a sense that if the horse stops making money they're no longer useful, but I really don't think at 23 & 24 after the careers they'd had, these horses were in that category. It's incredibly sad but their health deteriorated as they aged, would it have been better to let them suffer and struggle on for a few more months or years just because it was possible? No, absolutely not.
So YABVU about this. You don't need to know what was wrong with them, at their age it wasn't going to get better no matter what it was and for a bonded pair like they were, much kinder to let them go together than make one struggle on missing his companion while also having health issues.