People are talking almost as if phenolic is phenolic. Well, no. It's just a resin, and I'd guess there are many flavors of that resin. Furthermore, I know that phenolic composites come with at least 4 different kinds of reinforcing fibers. The ones I know about are paper, linen, cotton, and fiberglass. Presumably, there are others.
I also hear people talking about how much of what kind of stuff is sufficient for a level I, II, or III rocket, as if each level are all to the same design, go the same speed, etc. Design details matter, and material type cannot be used as a magic amulet. I could design you a carbon fiber composite rocket that would be guaranteed to break when using only a D engine. A shredding MMX in carbon might be a challenge, but possibly doable*. And I'm sure someone else with the pertinent expertise can design something in cardboard, paper and lumberyard wood that could withstand Mach 3**, though I'd guess for repeat use it might be wise to use something a bit more heat resistant on part of the nose cone and the leading edges. And the rocket nozzle, of course. Maybe paper phenolic?
I'm sure the material requirements for that gigantic Mars Lander enlargement someone made are far different than for something intended to get to 30,000 feet at Mach 3.
Ordinary materials with good design would be superior to just throwing G10 and carbon fiber at the problem.
Even for high altitude, a slow burner that remains subsonic can use THICK foils on the fins without a significant drag penalty. Like, 15 percent thick. If built with patience and a laminar section, these fins could be lower drag than almost all of the other fins you'd see, at least until the first bug splat or unrepaired ding. And certainly lighter and stiffer than solid aluminum or G10.
I am not a big rocket guy, but I've been interested in the engineering problems since I started looking into new fuel for Jetex and reading rocket forums. I have a background in mechanical engineering, have built several small, crude boats, and a variety of model airplanes, so I'm not completely ignorant.
*Especially if made from so-called carbon fiber veil and cheap epoxy.
**I'm sure I could design one for Mach 0.5