LineageOS[1] might let you wring another year or two out (formerly known as Cyanogenmod). I still have my old N5 in a drawer, so far I have been happy with my 6P...

1: https://wiki.lineageos.org/devices/hammerhead

I'd still be using my Nexus 5 if all the antennas embedded in the back plate hadn't started losing their connections (no wireless charging, wifi and GPS signal are super-spotty).

The beauty of the Nexus 5 is virtually everything is replaceable and an easy DIY project. Sure, you may not be able to get every part OEM, but I've extended the life of mine several times now (including replacing the entire rear housing) as it's still the best phone I've ever owned.

I replaced the back plate, cleaned the contacts, and such. No joy.

Are they connected with leaf-spring contacts? If so, and if you haven't already, try cleaning the contacts with isopropyl, the closer to 100% the better. RF is screwy enough to be magical, and if there's skin oil or something on the contacts, that might be enough to cause the issues you're seeing.

Something like that. They actually look like they might have coiled springs providing tension to the leaves on the phone side, and then contacts kind of laminated on ribbon cables in the back panel.

- contacts visible in back cover (the 4 yellow contacts for wireless charging are the easiest to see): https://i.ytimg.com/vi/-RWgjtEwQYQ/maxresdefault.jpg

- spring-loaded contacts in the phone (3 around the camera, 2 on the opposite top corner, 4 match up with the wireless charging points): http://images.anandtech.com/doci/7517/Nexus5-1552.jpg

In mine, the back cover flexes, and contact isn't always maintained for all the connections. I've cleaned them with contact cleaner, and alcohol, tried bending the leaves to provide more pressure, etc. I also replaced the back at one point. Like the original, it would be fine for a while, then need readjustment. The body of the phone has a few thin, brittle parts, which started cracking (the back really wasn't designed to be removed often). I feel like the wear+tear on the phone body and constant mechanical maintenance just wasn't worth it.

Prior to opening the sucker because of the wireless charging not working, there shouldn't've been any finger oils on the contacts (or, I'd hope).

The wifi connectors are something like MMX . The qxi charging coil is leaf spring. I still have my Nexus 5 after 3 years and it runs great. Would get a pixel but no wireless charging.