I got my forever bike in 2008, a 26" custom titanium hardtail MTB. It was a great bike for the time, but was built at the end of the line for 26" wheels, near the end of 1 1/8" straight steerers and 100/135mm QR, and we won’t get in to the geometry. I rode it regularly for many years (7-8?), but it was less and less my first choice. It sat mostly unused for a few years.
Beginning around four years ago, I experimented with converting it to dropbars, and since then it’s been through several iterations including different bars, brakes, drivetrains, forks and wheels. Most of these changes were difficult because of the outdated standards on the frame. How many decent rigid disc forks with reasonable tyre clearance and a straight 1 1/8" steerer do you think are around? What about wide tubeless rims with nice hubs and 100/135mm QR?
These days, and for the last year or so, it’s setup with dropbars, rigid fork (a c.2009 Kona P2 29er), 650b wheels (I laced my old Hope QR hubs into some new Stans Crest Mk3 rims) with XTR 1x10 and hydraulic Gevenalle levers. It gets as much or more use then my other bikes.
So, maybe there are forever bikes, but you may have to be flexible and hope it can change with your preferences and standards.
The original build, 2008
An awkward stage, 2017-2019
The current form, 2021