You aren’t wrong insofar as it is possible to hit weakspots on a bushed tank, but it’s more difficult than if it were non-bushed. The difficulty scales with the precision required to nail the weakspot. On an IS-6, like you mentioned, the weakspot is quite large and easy to locate bush or no bush, as it’s effectively the entire turret face to the right of the gun. Since he can never obfuscate where the barrel is, you always have an anchor to place your shot, just aim at the gun and aim slightly to the right.
But a more common example is the machine gun port on the Jumbo. The entire front of a Jumbo can be hidden by bushes (Particularly easily with the 6 bush pack), and the machine gun port is a small target that isn’t particularly close to anything you can use to anchor your aim. You can eventually find your way in through using the hitcam, but this requires a lot of shots, giving him ample time to respond, even if it’s only by barreling you. And that’s a reaction time he only has because he’s bushed, and I was unable to hit his machine gun port with my first or second shots.
This generally holds true for any weakspot that is either small, not close to any unhidable part of the tank, or bracketed by strong armor. Take the Maus posted just above. I know his armor is weakest where the turret normalizes, which is roughly in line with his guns. But I also know it has a mantlet at that same level that will eat my shot, and if I aim too far out, something like 90mm HEAT won’t kill his crew. And if he’s moving his turret, that’s a third variable. The same holds true for tanks like the T-54s, Tiger IIs, Pershings, Panthers and plenty of other tanks. And that’s just ones with vulnerable turrets. Turret rings are far easier to obscure. Yes, the hitcam will eventually let me put the shots where they need to go, but it hands them a massive advantage in the time it takes me to line up a shot I should have been able to make from the first second.
I’m not going to touch on the overall heavy versus light balance, since it’s somewhat off topic, except to say armor’s value is in giving you a reaction time advantage. If you have good armor, your opponent either has to take extra time to pick out a weakspot accurately, or they have to rush the shot and hope it doesn’t bounce. Meanwhile, if they’re unarmored, all you have to do is line up a shot center mass and click. All being equal, you will win that fight. The Jadgtiger is a bad example of this, as it’s binary. You can either punch through the entire casemate, or you can’t. There’s no need for enemies to take the time to aim an accurate shot, you either get lolpenned, or you bounce them without risk while they scuttle into cover. Tanks like the IS-6 are a better example. in that they have a weakspot that enemies can get through, but only with careful aim.