“Stable” doesn’t mean “doesn’t crash”, it means “low frequency of changes”. Debian only makes changing updates every few years, and you can wait a few more years before even taking those changes without losing security support while Arch makes changing updates pretty much every time a package you have installed does.
In no way is Arch more stable than Debian (other than maybe Debian Unstable/Sid, but even then it’s likely a bit of a wash)