Nothing, not even bacteria, can grow at 15% a year forever. We live in a finite world with limited substrate and positive-NPV projects, and all companies (even today's fastest growing ones) will eventually reach their growth limits.
Every company goes through a life cycle. In the beginning, they grow slowly, if at all, as they just get off the ground. Then, they enter an "exponential" phase during which they experience their greatest growth. Then, they hit their "maturity" phase, where they bob-around at the same level or afterwards decline.
In the distant future, when every man, woman, and child on Earth drinks 3650 Cokes a year (10 a day like me), Coke will hit its maturity phase. But we want to come up with a growth rate that will summarize Coke's various growth rates throughout its entire life.
Given Buffett puts the Washington Post's growth rate at 6%, it's reasonable to assume Coke's is higher, say 8%. Yes, it could be a little higher or a little lower, but that is the hard part -- estimating the "coupons" you should plug into the perpetuity formula. Buffett wrote,
"In The Theory of Investment Value, written over 50 years ago, John Burr Williams set forth the equation for value, which we condense here: The value of any stock, bond or business today is determined by the cash inflows and outflows - discounted at an appropriate interest rate - that can be expected to occur during the remaining life of the asset. Note that the formula is the same for stocks as for bonds (meaning perpetuities). Even so, there is an important, and difficult to deal with, difference between the two: A bond has a coupon and maturity date that define future cash flows; but in the case of equities, the investment analyst must HIMSELF estimate the future 'coupons.'"
Should you plug in 0, 1, 2, 3, 4, 5, 6, 7, or 8% as the owner earnings' growth rate, g, in the perpetuity formula? No kidding it changes your intrinsic value estimate dramatically if you change your growth assumption. That was the whole point of the "Some Valuation Math" section in Buffett's 1991 Letter. But that is exactly what the investor has to do or else they have no idea of intrinsic value and are consequently investing "blind." I use the Washington Post's 6% as a proxy for Coke (i.e., the growth rate of Coke, the best company in the world, should be higher than 6%, say 8%).
You will not be confused if you distinguish in your mind short-term growth rates from long-term growth rates. It is the long-term growth rate you plug into the perpetuity formula, not the short-term one, which is only transitory. As I told you before, if you expect Coke's owner earnings to remain 20% a year forever, then by all means you can conclude that Coke's intrinsic value is infinite. Or if you take 20% to be a short-term growth rate, you can do a two-stage DDM followed by a one-stage. But the perpetuity formula is sound (for long-term growth rates), and ultimately I trust Buffett more than you.