Very interesting topic, I loved reading everyone’s thoughts on this. I very much agree with Suntiger’s three points above.
For myself, I do aspire to longevity in my wardrobe, and generally tend to keep things for a long time (even if not necessarily "forever"). I have noticed that there are certain garment categories where I tend to have more success than other’s in finding long lasting pieces though, and where I would willingly spend more money, namely coats, knitwear, bags and smaller accessories. I have several wool coats (some bought new and some vintage from the 60’s and 70’s) that I’ve had for more than a decade and that I can’t imagine I will ever get rid of. I have knitwear that my grandmother knitted for herself in her younger days that I have been wearing since high school and that still looks as good as new (while others have not aged quite as well, like some oversized 80’s acrylic sweaters), and simple crewneck cashmere sweaters bought vintage that I’ve had for years and years. The bags that I wear daily are also all more than a decade old (both new and vintage). Other garment categories I don’t expect to last quite as long, like tops, trousers and shoes. Both because they get much harder wear, and also because those are pieces that I tend to use to update my style and keep it more current - like changing trouser silhouettes for example.
I think in general it’s getting harder and harder to find that forever quality though, no matter the price point. I don’t think it’s a coincidence that most of my oldest (to me) pieces are vintage. The last thing I bought new that I feel has the potential to be a forever piece was a forest green wool and cashmere coat I got in 2015 that has been in heavy rotation every winter since. It was a true impulse purchase that cost around $500, and I think it was part of the debut women’s line of a local brand, so they had a very small production at the time, which I think contributed to the high quality. That coat has been part of every winter collection since, but sadly the quality looks worse each year while the price increases.
Like some others have said, I think those forever pieces are often serendipitous, but if I were to plan for them there are a few things I would consider: fabric (natural fabrics, and thicker, sturdier fabrics tend to age better), production (like I touched on above, I think brands that have a smaller production can produce better quality), knowing my own taste and the parts of it that has stayed consistent over the years, and if it’s not an iconic/statement piece I would try to aim for a degree of timelessness and restraint by avoiding exaggerated shapes (like very oversized or fitted, too flared, too strong shoulders, etc), and too much detailing.