Why is the continued operation of the International Space Station eventually deemed non-viable?

It is an aging machine where maintenance complexity and spare part availability decrease

The operational viability of the ISS is limited primarily due to its age and complexity as an aging machine that has been continuously inhabited for over two decades. Many of its original components were designed for a significantly shorter service life. As systems age, the inherent likelihood of critical failures rises, compounded by the fact that sourcing spare parts and specialized expertise for decades-old technology becomes increasingly difficult. Furthermore, financial considerations weigh heavily, as the substantial cost of continued operation becomes less viable compared to investing in newer, potentially more efficient commercial Low Earth Orbit (LEO) platforms.