I guess I'm far from brand loyal, but I don't keep changing my brands, just adding to them.
On the digital interchangeable lens front:
My DSLRs grew out of having Pentax SLRs (& lenses). I tried the K100d & liked the feel & features of it, apart from the price at the time. Several years later I brought one. Subsequent upgrades where due to new features, or to further expand my lenses. In this time I've assisted several friends with their consumer Nikon/Canon DSLRs & always been disappointed with them in comparison to my bodies.
At one point I wanted to try tilt, but the price of a proper lens was excessive. I spotted a tilt adapter was available to use my lenses on MFT, and that this + a used body was a tiny fraction of a tilt lens, so added MFT to my selection. Subsequently adding some native lenses to make the most of it's small size.
After years of using both these systems with an increasing hoard of adapted lenses I added a Sony NEX6, enabling me to adapt to a larger sensor, as well as giving other advantages my existing mirrorless bodies didn't offer. I've now added an A7ii but despite having 2 Sony bodies I haven't got any Sony lenses.
On the film front I have SLRs with at least 4 different mounts, several mounts having examples from multiple manufacturers , as well as medium & large format stuff. No brand loyalty there either.
The biggest surprise is how little of my gear is Canon or Nikon! I have a couple of lenses in each mount (pure chance they came as part of a job lot) & a fancy canon cable release (works with Pentax). other than that it's only adapter combinations that go via EF