-Changing art doesn't make things currently unpassable, passable.
Actually I believe you are wrong about that. There have been hacks in the past that have allowed characters to run through things like crates, candelabra and bag balls. Maybe they don't work anymore, or maybe I was misinformed and they aren't art hacks, but historically there have been hacks that allowed players to run through objects of that nature that would normally block movement.
Having said that, that was only a small example of the kinds of advantages art hacks offer. There are plenty of other real, and unfair, advantages offered by art hacks, such as detailed and clear labelling of fields, and much more.
-A simple change in your .cfg file will allow you to walk through gravestones in Felucca. This is not a hack in anyway.
I wasn't talking about that. I've heard that that configuration change actually doesn't get rid of the tombstones, so I'm guessing they will still block movement. But because they don't appear in the enhanced client at all they should probably just be gotten rid of. They do offer an unfair advantage to those using the enhanced client.
-Changing art allows even the most modern computers to handle dozens of field spells and players on the screen with a lot less cpu loss. (less lag)
This is flat out nonsense. Art hacks in the classic client offer absolutely no performance advantages to someone playing on any reasonable gaming computer, and certainly any desktop manufactured in the past five years. This is typical of the kinds of excuses cheaters make for their behaviour.
-Speedhacking and the enhanced client accomplish the same thing. They allow people to skip and jump across the screen. This requires turning, straight lines never matter. It's not simply obstacle collision avoision with enhanced client.
Again flat out nonsense, but I won't repeat everything I've already posted on that topic.
-'Hacks' as they exist in UO, are rather benign. In most cases they are done to compensate for the 13 year old clients' failings. Annoying at most.
That's actually pretty funny. That's exactly the same argument that those who supported the use of scripts have made over and over again. The truth is they offer real advantages not available to those who refuse to use them because they are a violation of the TOS. If they didn't offer real advantages, no one would use them. Fields that are clearly labelled, colour coded, and much easier to target than those using unmodified art files offer a clear, and in some cases, decisive advantage. And that's just one example of the kinds of advantages offered by art hacks.
-There's too much talk about hacks, but nothing about exploits. Exploits are the silent killer of video games. Duping, the skill bug, old house break in bugs.. exploiting the actual game through gameplay.
Both are destructive, but I'd argue that hacks are probably more destructive in the long term because they they don't get fixed the way exploits do. All those examples of exploits that you listed have been fixed, and will continue to be fixed as they come to the attention of the developers. Most of the art hacks have been with us for at least a decade, and the only "fix" for them will be client file consistency checks.