Yes, grass, alfalfa, whatever plant loses nutrients over time(often quite quickly) but unfortunately, sugar is not one of the nutrients that it loses - can be just as high in year old hay as the day it was cut!
Sugar is not a problem with alfalfa however.
I live in Arizona and alfalfa is practically all that is available/affordable. I don't think I ever asked about protein content on hay nor do I think any of my hay sources would even know. One place just looks at me blankly if I ask what the bales are running weight-wise.
Anyhow, we (and all our neighbors) pretty much feed straight alfalfa and have no issues. I wish we could get a good alfalfa/grass mix but grass hay is so darn expensive around here. And the quality can be inconsistent as well. At least alfalfa is available in good quality.
Anyhow, my horses have great feet (trail ride in the mountains with boots only as needed), great weight, no problems. Straight alfalfa may not be ideal, but it isn't the boogey man most make it out to be.
I am a firm believer it is a very good hay to mix with grass hay for the protein content.....if you have access to grass hay. In fact, I would consider that ideal.
Now grass pasture I would consider scary. I wouldn't have any idea how to keep a horse on pasture.....I would worry about the sugars. Isn't that funny? Most of you have a comfort level with pasture and I have a comfort level with alfalfa.