Volumes likely won´t grow (think in the past decade,organic volumes have increased by less than 1%/yr), and if they do, it will be minimal.

Not an unreasonable observation. Volume growth is not their thing, partly because they are so large already in the US and more because they are seemingly not well suited to organic international expansion.
Volume growth is unlikely to pick up a lot other than with the potential of some acquisitions, historically a modest factor.

Two notes, though.

First, sometimes I wonder what constitutes a good number in the business of branded junk food : )
Coke is a darling of many, but their real top line revenue has fallen -3.2%/year in the last 9 years.
That makes Hershey's top line growth look not so bad at inflation + 1.8%/year in the same stretch.

And second, one can do very well financially with very modest volume increases. It depends mainly on the quality of the capital allocation.
Coke has managed real EPS growth of -0.2%/year in the last 9 year and +1.8% in the last 15.
Hershey has managed real EPS growth of inflation + 6.4%/year in the last 9 years, and inflation + 7.7%/year in the last 15.
At constant valuation multiples, that rate plus the dividend yield will be your real total return. So they've managed about the same or maybe better than Berkshire in the same stretch.

I'm a lot more optimistic than you, partly because I'm focused a bit more on the per-share financial side of things as well.
You're expecting growth of 3%/year, essentially zero in real terms. If that is also taken as the financial expectation for shares, the real total return would be just the dividend yield, around 2.65%. That's a real return as it is likely rising with inflation.
I'm expecting value generation of (say) 5-6%/year more than that. Value per share growth (real EPS growth) of maybe inflation plus 5-6%, plus the same real dividend yield. Plus a moderate one time bump from valuation levels.
Maybe EPS will grow a bit more slowly, but it still seems like a reasonably attractive return given the solidity of the company.

Financially, I find the omens good. ROA and ROE of ~25% and ~45% respectively sitting on top of book per share growth over 12.5%/year in the last decade. Net returns like that bespeak a moat. No moat lasts forever, but despite the worries on competition, the business they're in isn't really changing much. Sweets and snacks remain popular.

Jim