When my husband and I were shopping for my engagement ring, my husband asked about 'investment quality' stones. The gemologist (I was in the business so this was really on the up-andup) said that wasn't the way to shop for an engagement ring. An investment stone shoudln't be set and shouldn't be worn. An engagement ring is meant to dazzle the eye and make you happy to wear it.
I think its the same with bags. The idea of keeping a Birkin or a Chanel for more seasons than you would an Isabella Fiore is just that one has a more durable style and is less beholden to trends. Its not a question of holding its value. Yes, it holds its value if you get 30 years use from your Birkin and 18 months use from your Fiore, but that's why a Fiore is less than $1k and a Birkin is impossible to get even if you have the $.
Also, some bags become precious 'keepers' because they aren't every day occasion bags. I'm sure I'll run my Fiore into the ground soon enough -- I carry it in the rain, I overstuff it, I'll even toss in a quart of milk that I buy on the corner. My limited edition silver Paddington -- I'm not going to treat it so roughly so it will be around longer.
Is anyone buying a bag, not because they love it, but because its needed to fill out the 'collection'? Like a museum 'needing' a Picasso and a Renoir and a Degas, etc, etc.