The 1 IP/turn semi-auto marksman is obviously an exaggerated example. It doesn't change a thing about the fact that according to the stricter interpretation of the rules, if two equally fast people fire a Tiffany Needler and a Panther XXL side by side at the same rate of fire, only one of them will suffer from recoil penalties. And that completely flies in the face of all reasonable expectations.
Then why not apply Strength requirements to be able to fire an Assault Cannon? I mean, you're lifting it so Strengthx5 would apply to how much weight you can manage. Seems silly to change recoil rules for all weapons based on Heavy Weapons players being problematic. Also, if your GM gives you a hard time for having a dozen assault rifle magazines with you (honestly, 3 on your body and 9 in a bag would be perfectly fair, and it's not as if you ever need more than 4 clips in a single gunfight), why would they not give you a hard time for carrying an Assault Cannon with a lot of ammo for it with you?
"Saying the problem can be discounted" <--- I do not know what you are talking about. Can you explain exactly what you mean and where I said something that you consider to be saying the problem can be discounted?
Action Economy: A Sniper Rifle costs anywhere between 10k and 28k so their cost makes any ammunition costs irrelevant, especially once we start modding them with "Weapon Cost" modifications. A sport rifle is as expensive as an Ares Alpha and does a bit more damage, but cannot fire bursts of any kind and thus also cannot use Suppressive Fire, meaning it hits less against high-dodge enemies. Trust me, I missed burst-fire options when facing Shedim. So while yes, a gunhappy gunbunny will spend more on ammunition, they get to tag their enemies far more easily. The longarms user, on the other hand, eventually has to splurge on a sniper rifle which nullifies all ammunition costs the Automatics user might have.
As for your Longarms arguments, let me tackle those one by one.
- The advantage of range and surprise applies to both longarms and automatics. At 51~150 meters they both are in Medium Range. So that isn't a significant difference between the two.
- Choke settings come with a quite significant loss in damage and also are only useful when enemies bundle together.
- Furthermore, they're only available for shotguns with flechette ammo, which makes lethal shots impossible. So vs Drones and Spirits this has zero value.
- Shotguns have really lousy Accuracy and are worse in range than an Assault Rifle.
-> As a result of the above points, choke shots are only really useful in close quarters.
- The automatics expert can apply Suppressive Fire instead, this will do much more damage.
- BF is available, yes, on a shotgun with 4(5) Accuracy where you must decide between easy ammo switching or a decent clipsize. And that shotgun is useless at very large ranges, unlike an Assault Rifle, and has no FA options.
As such, I do not believe Longarms have enough advantages left over Automatics under a ruling where accumulative recoil does not ever apply to Simple Action firing. Defense penalties are far more powerful in SR5 than in SR4, so Accumulative Recoil helps balance out Automatics.