I think the main issue with spikes is just the variety of terrain a trail shoe can (or is designed to) cover. In the north east where I lived…or even Texas this year…snow and ice, mud, loose sand, tree roots, boulder fields, ankle breakers, tree roots, and pavement were all available on nearby trails. A rubber knobby tread is a compromise on all of those, but also capable of handling it all. Metal spikes would fail miserably on pavement (even just road crossings), boulders, and likely mud/ loose sand. This is where the soloman/inov8/any other fell running style shoes shine and most trail shoes outperform roads. There are also a few options that DO have metal spikes built in for “off trail” such as the oroc from inov8 or spirit from icebug.
I’d also be willing to say if you don’t feel the need for any additional traction over road running shoes, the conditions and trails you are running on are quite mild. I see a lot of people here in North Texas running on trails in road shoes, and would consider it myself if I didn’t have a closet full of trail shoes. Trails are typically closed when wet and conditions are just hard packed dirt with a little sand when dry and open. Road shoes with a little tread are plenty capable of handling that.
On the removable cleat part, this already sort of exists with yak traks, microspikes or screws in the winter, depending on how loosely you want to interpret it. I would not want a permanent solution for the weight concern or general reliability. There’s a very large difference between unclipping and walking a mtb up a technical hill and bombing a downhill while running. The last thing I want to worry about while bouncing through a boulder field is if my cleat will wiggle a bit causing me to slip or something worse.