I've found it difficult to answer this levelly, because your post is (to me) so clearly offensive, but I'll try.
Why do you think someone with 4000 karma is no longer a PC, is a functional god, whatever? Considering that the potential difference between one character with that and a GM's NPC is ... what? Nothing. Because this is Shadowrun, and because anything that can be acquired/learned/used by one character can be acquired/learned/used by another. 13 Agility? Sure, that's just a standard elf (or Gnome, Hanuman, Menehune, Dryad, Wakyambi, Xapiri Thepe, Oni, or Falconine, Lupine, Pantherine, or Tigrine Shapeshifter, or Nosferatu, Harvester, or Banshee) with both Exceptional Attribute and Metagenetic Improvement in Agility (or Nocturna or Pixie without one of those two) who's had the attribute maxed out, plus maximum enhancement. Have a 13 skill? Hey, PCs aren't the only ones who get good, and as the man says, "There's always someone better than you." Or at least as good.
It is difficult for me to imagine being ABLE to bring all your gear on every run, whether you're at 4/40/400/4000 karma or not, whether you've earned 10,000, 100,000, 1,000,000, or more nuyen. If a character's story is not done - and until they're dead, they are always potentially unfinished - then why could you NOT return to the character and play them some more? Certainly new characters are played, whether that's because a prior one has died, you join a new group, or you just get curious about a different ability or play-style or whatever. And of course, here's the amusing thin - I am wondering, why do you restrict your real self artificially from developing a character's complete potential? Do people just stop when they hit a certain level, or do they keep trying? "Oh, I made manager, I'm done." Some, sure, but let's face it, in a non-level-based RPG you develop side skills, you get into other things which is where your XP/karma goes, you spend it to learn something you desperately need at the moment (like that language at a 1).
Likewise, what you are calling 'artificial limits' are imposed by your subscription to the universe of the game in which you're playing. If there are (as has sometimes been conceptualized) 300 street samurai / physical adepts / muscle in Seattle's 3 million, and this represents a good ratio worldwide, then what comes of the journey of the samurai who wishes to be among the top ten of those 700,000 non-aligned individual-mercenaries-for-hire when one shrugs and says after a hundred or a few hundred karma, "okay, that's enough, we're too powerful"? Until you have your weapon skills and gymnastics all maxed out, and you've encountered and fought individual conflicts by the score, and you and your GM have played your storyline through peculiarities and injuries and all sorts of things ... where's the story?
What I like about Shadowrun is the same thing I enjoyed about 1e Warhammer Fantasy Roleplaying - given the most simple of circumstances, a first-day security guard (city guard in WFRP) can flat-out kill even your most advanced character. Is it likely? Hell, it's not likely for starting characters to be so slain, but no matter the amount of armor, or skill, or anything, a nervous guy with a shotgun can still Ruin Your Day. Why? Because what you continue to call 'artificial limitations' are what so many of the rest of us call 'the enforcement of reality'.
I think I would understand you better if I could understand what your 'character path' usually winds up being, start to finish, and what sort of social contract exists between you and your GM in regards to the world within which you play your characters. You start out with what, a low-end shotgun or pistol, an armored jacket, and a motorcycle, and you build yourself up until with every mission you're carrying an elite assault rifle firing high-velocity APDS/AV rounds, an under-barrel grenade launcher (with a wide variety of munitions), wearing heavy military-grade armor with all the fixin's, carrying ATGMs and C12 with a remote Thor Shot on standby ... and then what, you stop playing that character, because there are no more challenging opponents? Do you always go into every mission brining the heaviest weapons, armor, and gear that you have?
Serious question, this: what kind of game are you playing??