But when you say "The Arts", that might well be very flawed, very incomplete - and now it's "too late to fix" (don't ask me why).
"Improved" may not be the right word.
As I've always understood it, there were (and still are) many, many different theories and understandings of magic, and they vary widely and wildly. Three picts grooving in a cave, and they come up with their own understanding of "magic" - that has no relation to anything happening in the next valley. Some may have been close to what is "now" Hermetic Theory, or was in part, but for each that was there were others that were not. Even in 1220, each non-Hermetic Tradition varies, and some were more or less powerful in what Hermetic magi know as the "Forms" or "Techniques", but they worked well enough. It was "magic", even if it was all different magic.
(This explains some of the "weak" powers and alternative magics that are suggested in supplements - it's not that they are attractive when building a Player Character mage, it's that they are all some NPC Hedge Wizards have to work with. And why some of the really cool suggestions come with painful restrictions - again, these aren't "splat books", they are alternate approaches to magic that some non-Hermetic magi practice.)
What the big B did was find an underlying element that could explain all of them (more or less), a magical GUT. Only it wasn't perfect. In some cases, significant abilities were lost - some of the powers that are found in Ancient Magic, Mysteries Revised, etc. are examples of "Theories" that encompassed some understanding that was lost when translated into Hermetic Theory. Some of the sharper corners had to be rounded off to get it all to fit into that one package.
The power of Hermetic Theory is not that it's "better" - it's that any and all members use the same theory, and that it covers 98% of what's out there. It's the Volvo of magic - sturdy, reliable, works well enough for most any purpose. But it's not a blanket "improvement", not at all.
That's how I understand it.