The longest-lasting Studio desktop was the Surface Studio 2, which was released in 2018 and wasn't replaced until a revised Surface Studio 2+ was announced in late 2022. An even higher quality display panel was used, but it still used earlier-generation internal components. This might not have been so egregious if Microsoft had updated it more consistently, but this model went untouched for so long that Microsoft had to lower Windows 11's system requirements specifically to cover Studio 2 so that the company could continue to support a PC running it still actively sold.
The Studio 2+ was the last hurray for the desktop, and despite jumping two GPU generations and four CPU generations, it still didn't utilize the latest components available at the time. Again, more consistent updates like Microsoft's for the Surface Pro and Surface Laptop could have made this less of an issue, but the Studio 2+ remained untouched for another two years after the update.
The Studio desktop's unique screen and hinge endeared it to some artists, and for those users there is no immediately obvious replacement for this machine. But the high price and specs of the all-in-one always made it a hard sell for anyone else. A lack of broad appeal usually leads to mediocre sales, and mediocre sales usually lead to discontinued products. That's how it goes.
