Here's the partial list.
Cracking varnish everywhere as it hardens, starting with the edges and corners. I think keeping down the number and thickness of individual coats may be helpful in this regard.
Relaxation of the stretch, with ultimate complete fatiguing of the canvas after many stretches. Up to a point stretch can be restored with corner wedges, but after a few wedge adjustments the entire canvas will need to be re-stapled to avoid over stretching the corners. Failure to re-staple is why so many old canvases have rippled corners. Repeatedly using the water spray stretch trick may be not a good idea over the very long term.
Differential shrinking and swelling of the individual layers of canvas, receptor, ink, and varnish.
Exposure of the unprotected canvas back to air pollutants and humidity cycling. One can coat both sides of the canvas before stretching, maybe that would help. Or maybe that would just make the canvas stick to the bars.
Foxing of the canvas from chemistry in the stretcher bars. The commonly used pine stretcher bars are the biggest offenders, but even kiln-dried basswood is still reactive to a degree. Varnish the stretcher bars? Maybe. Or perhaps use archival isolation tape.
Warping stretcher bars. For enough money very stable bar systems can be purchased if you have the budget. But affordable stretcher bars will eventually warp.
Physical damage. A lot can happen over the years. I have many canvases installed in a local health care system, a lot of them have taken serious hits from various rolling medical devices.
And of course fading for ordinary inkjet prints. Canvases often wind up in public places with UV-rich fluorescent lighting. And even for fade-proof inks, localized heating from spotlights aggravates mechanical issues.
I made that 50 year quip because I recently looked closely at a lot of Taos School paintings from the early 1900s. Most of them were in deteriorated physical condition after less than a century in relatively benign environments. The biggest issues were desiccated canvas backs that made further stretching risky, canvas tears that had developed in situ, rust spreading out from staples, and chemical edge foxing and physical abrasion from stretcher bars and wood frames. Add to those amateur repair attempts that had simply made matters worse--think packing tape and Elmer's Glue. Those issues have effective but very expensive conservation solutions.