As with many things, I think this massively depends on where in the world you are and what forages are available to you. I live on the prairies and many times we will only get one cut of hay on non-irrigated fields - often in late August or September and as a result it tends to be quite mature. This has been fine to feed to my crew of easy keepers free choice (including a fjord and an andalusian who get fat on air). That said, I also have a nearly 40 year old, and when I had to switch her to soaked cubes I struggled to keep her at an appropriate weight on roughly free choice as the only cubes available in my area are 50% alfalfa. She was previously on hay with the others and maintained a decent body weight. We also have space for them to move though, which is a lot different than just parking in front of a pile of hay in a stall.
Do you ever feel like life is a balancing act between not oversimplifying things while not overcomplicating them either?
Owning animals is definitely like that. It’s easy to fall off either end of the spectrum.
We all should be striving to keep our horses in good health while also recognizing when you need to compromise. 
“Forage” means they are foraging for food. Standing in front of food is not foraging. I have easy keepers and hard keepers here. I have seen quite a few horses have to be put down because they were over fed and became metabolic etc. These were not old horses and to a horse they had hay 24/7. One size does not fit all, it’s up to us to see this before it’s too late. If I knew how to post pictures I would post the three Fjords in my care over the years.
I also have a PRE and a Lusitano here. They are roughly the same size and weight. One is a 5 and the other one a 6…the 6 has to get fed very differently because she was an 8 when she arrived. So the charts can’t tell you how to feed…hell one of the feed companies tells you what to feed as a min mum, that cracked me up because if I fed the PRE the minimum they would be fat.
I keep hay and or grass available to my horses pretty much all the time. I muzzle them when the grass is too rich (especially when the cool season pastures are lush) or the horses are gaining weight. I put the hay in slow feeders as needed to control consumption.
I make the horses clean up the hay that I give them (within reason).
When I got my pony he was overweight, so I did have to ration his hay strictly at first to get weight off him. Now that he’s slim, he doesn’t have to be rationed as much.
I feed warm season grass hay, which I think helps. My two middle aged to elderly geldings play like colts. I suspect that helps too. The pony is the elderly one, but he runs and broncs with the best of them 
Some horses are easier to manage than others. My dearly departed Norman was very difficult. My current two are simpler; hay, water, ration balancer and good to go.
I recently read an article in a vet publication, I wish I could remember which one…
The gist was that horses shouldn’t go more than 4 hours without forage. So if you feed last hay at 10 pm , for example, they’re done by midnight, then by the time they digest it if they go without until 6 am or whenever you feed breakfast then that’s more than 4 hours and could cause a problem.
Mine are out probably
75% of the time, I feed rolls in the field, and squares when they are in during the day from 11 am until about 5 pm because Florida. They get it free choice 24/7 so no empty bellies.
My horses go 4 hours with no hay in the middle of the night/early morning. I have never had a problem so I am not sure where all this is coming from. Bottom lines a fat horse is worse than a horse going without hay for a few hours. I have bad feet and can’t imagine being a horse and dealing the hoof pain because people think wrongly that the horse needs to be eating 24/7. When I was a kid there was not this epidemic of fat horses…maybe the feed companies have done what the fast food companies did only to horses.
I think you’ll find the agricultural definition of forage (noun) is feed that is not grain/oil seed/etc. It can be anything from grasses (including grain grasses) to legumes (alfalfa, soy, peanut) to grain straws that is somehow preserved (dried or fermented). It is not specifically anything an animal may have to forage for in pasture or through browsing.
That said, I have NO, absolutely ZERO, argument that over feeding horses is detrimental to health and longevity.
I am of like mind with you on this. I don’t know when it all started but I really don’t understand the thinking behind it. Whatever the reason people are falling for it hook, line and sinker .
When using a new type of feed, I weigh it in the scoop I will use to dish it out, on a scale kept in my feed/tack room. After that, I needn’t check it every time I feed, as I’ve ascertained how much a given volume weighs.
I have a hay scale in the hay shed, and occasionally weigh a hay bag, mainly to keep my eye and feel dialed in. I’ve gotten pretty accurate at guesstimating hay weight, and needn’t do this anymore the vast majority of times. I think if someone hasn’t been weighing feed/hay, doing so will be eye-opening. Back when I boarded my horses, if anything, hay was skimped, not overfed.
As far as my horses’ weights, I’ve found that the commonly available weight tapes are very accurate as compared to the electronic scales typically found at vet offices, therefore I find them useful (but I can understand accuracy differing with breed, horse build, etc.). For example, one horse always weighed between 775 and 777 back when he was very regularly worked, no matter which electronic scale, same as the tape.
Took him to a new clinic mid-winter after he’d been semi-retired for a while, and told the attending veterinarian that my horse had gained a few pounds, but was still under 800. Bingo, he weighed 790 on the scale; he’d gained approximately 13 pounds (counting his winter coat).
