While I already had a Pinephone in-hand I could have continued with the N900 to this day had I not gotten the deballed SIM issue.
While I cannot access most websites with the browser on the N900 I can play audiobooks, make calls, send SMSs, connect to 3G data and share it via bluetooth, and I can also connect to my Pebble smartwatch.
Only recently has Pinephone/Mobian gotten a good audiobook player, it can make calls/SMS, the bluetooth PAN wireless access point is not ready yet(though it can do WiFi hotspot), there is no libpebbe app to use the wrist watch, and the GPS navigation is not working correctly right now. Most importantly I can expect a system crash and reboot around once every 24-48 hours.
So yes, If I had a working N900 I would probably still use it as my everyday phone, but I guess it is good I am working on making Mobian better.