I answered in your post as direct answers in red
Most people would make claims of a racing fit Thoroughbred being to thin...no, they are race fit and trim.
It is far healthier for a horse to not lug around extra weight on legs often pencil thin bearing the weight of their massive body.
Regardless of age a horse carrying proper weight is far healthier a animal to fending off aging issues of stiffness and organ failures.
I would far rather my old horse being a bit under who has a good appetite I can feed more to in winter and blanket them as is needed to keep them toasty comfortable for them.
If you actually take the time to look at several "how do you score a horses weight"
websites you will also see a difference in photos accompanying descriptive blurbs.
Some sites claim 4.5 - 5 is what you want to have your horse at and then other sites actually say a 6 is a minimum where you want your horse at.
Few horses can really carry the extra 100 pounds safely that tips them to a 6 on the scoring charts I was taught to use.
You tip a bit to far you will find yourself with a ill horse or well on the way to having one...
Forget the added strain to legs, heart, lungs and all connective tissue...
For the same reasons...
What is healthier and less inclined to health issues in humans...the person thin or the person "fluffy or padded" aka fat?
Much though does have to do with being slightly overweight and physically fit or being fat and out of condition and that also goes for the one thinner but fit or thinner and sickly...takes many pieces of the puzzle to make a determination honestly.
Do you think that we, as horse people, are generally more "used" to seeing overweight horses? Absolutely.
I'm kind of curious about your opinions. Obviously, you want a horse to have good weight and good, even muscle. But which is the lesser of two evils: slightly underweight or slightly overweight?
If I had to choose, my horse would be slightly under not over weight
For a majority horses, the consensus is a 5-6 ish body score. So, would you rather have a 4.5 ish or a 6.5 ish body score?
Unless this was a breeding horse and actively covering mares or a mare pregnant carrying I prefer 4.5.
Otherwise, actively breeding or pregnant then yes a 6.5, but as a riding horse...no.
There are major health problem in overweight horses. Again, while you want a horse to be a good weight and muscle, what's wrong with a thinner horse?
There is nothing wrong with a horse whose ribs can and are seen when it moves, when it breathes and moves.
There is though a difference in fit and a horse in good flesh and one who needs weight.
As long as the "markers" used in scoring a horse are met then fine.