Plenty of monitoring estates in the UK are now orphaned. The devices are still on the walls and the data still arrives, but the company that supplied them has moved on. Hardware is discontinued, support has wound down, and nobody is writing a roadmap.
Nothing fails on the day the vendor walks away, which is exactly why an orphaned estate is easy to leave alone. What degrades is the repair path. Spares stop being available, firmware issues stay unfixed, and when a hard failure does arrive there is nothing behind it.
The useful window is while the old system is still working, because a changeover planned in that window runs to a timeline you set and costs considerably less than the same work done in an emergency.
If you have nothing to replace and are looking at monitoring for the first time, the solutions pages are the better starting point. This page assumes a system already in place.