Salt is more fundamental for the body than just flavoring. I'd hesitate even to call it a spice. That said, umami ingredients like mushrooms and seaweed are certainly used together with salt.
There's a long history of glutamate containing food and food products being used in place of salt across the world, before it was first chemically isolated.
Well you were replying to a comment about why we foraged for them before salt broski. Doesn't really make sense to bring up the price of something that wasn't isolated then.
I'm skeptical of any food that humans only started eating since the industrial revolution, including those that are derivatives of or isolated compounds of real food. Mostly the effects on our bodies are not well studied. I haven't specifically read studies on msg though.
I mean, foods like soy sauce and yogurt and sauerkraut predate industrialization, but are very processed. I wouldn't worry too much about MSG in particular, since it is also mostly made by fermentation:
