When the old one is no longer suitable for use as a primary phone:

• Nexus 5 - broke the screen. Actually still mostly works to this day, and I have Android 9 on it.
• Sony X Compact - small onboard storage, Android's crappy SD card support, and a lack of quality third-party ROMs after official support ended shortened this one's service life.
• Pixel 4a - current phone, don't want to "upgrade" to something that's bigger and doesn't have a headphone jack, mildly annoyed the fingerprint sensor broke.