That said, I almost bought an FM3a after college just to support the complete lunacy of building them new. At that point, I didn't really have enough Pentax lenses to be wedded to the system. I still kind of wish I had.
You got me thinking though: since you can adapt basically anything to E-mount, the cost of trying out another line of lenses is now down to <$100 US (cost of adapter), at least for manual focus and aperture lenses. A well-designed mirrorless mount and adapter basically renders the concept of a photographic system obsolete.
For those unfamiliar, the key dimensions on a mount are the flange distance (distance from the plane of the mount to the plane of the film), and diameter. There are two things to know about flange distance:
1) It used to be "large" (45mm give or take a few) to allow for room for the mirror in a DSLR to swing upwards.
2) You can't adapt a lens with a smaller flange distance on a body that has a larger flange distance. Well, you can, but you use infinity focus.
Because mirrorless cameras don't have a mirror (go figure), they're freed from constraint 1, and can make the flange distance as short as they care to, basically letting them eliminate the issue of 2.
Adapters used to be awkward, at best, (nonexistent at worst) because they would have to adapt two mounts of fundamentally similar diameters without having a lot (any?) room to move the lens along its axis.
Adapters for a mirrorless body can offset the lens tens of millimeters along its axis from the body flange, leaving plenty of room to resolve any mechanical incompatibilities in the mounts.
And the mirrorless bodies have really led to adapter heaven!
I found that those things mostly just stressed me out. I'd be much happier paying a hundred bucks a month for access to a darkroom where I didn't have to worry about those things and could just enjoy the process of developing and printing.
I've never been more than a casual photographer, so figuring out how to run a darkroom by myself and how to make enough use of it to be worth my while were too big a hump to get myself over.
I love the magic of watching prints develop and even the smell of a darkroom, but truth be told, getting a full-frame digital camera recently (a Sony a7ii) that I can put my old lenses (screw mount Takumars and Pentax M primes, one Pentax A zoom) on has done more to rekindle the joy in photography for me than anything.
The Takumars, in particular, are maybe the most beautifully made things I own. Oh my god, the knurling of the focus and aperture rings is unbelievable. The action of the focus helicoid is divine. We will never see lenses made the likes of them again because the machining costs alone would probably double the price of the lenses and the beautiful action of the focus helicoid is probably completely at odds with fast autofocus.
I will concede that from an optics and coatings standpoint, the world has moved on in the last 50 years. And obviously autofocus is a thing now. But I'm not a good enough photographer to be hitting the limits of my lenses, and for me part of the joy is the way they feel in my hands.