I can give you my opinion, but understand that just as many people feel 100% the other way, and that's okay too.
I've seen 80 year old airplanes where the inside of the wing or tailcone look like new. Maybe a little dull, but no corrosion since the day they left Wichita. Remember that of all the zillions of Cessna's out there, none of them had internal primer until probably some time in the 70's. For a while there, it was an option that you had to pay extra for, or it came as part of a seaplane provisions package or something, but mostly no internal primer whatsoever. and they were fine.
But I've also seen airplanes that were corroded like crazy inside. Mostly, the problem seems to be with water intrusion or a salt air environment. Sometimes God forbid, bird droppings or mouse urine (that stuff will also destroy steel control cables)
On my 7, I'm going middle of the road and priming the mating surfaces inside the flight controls, wings & tailcone with either zinc phosphate in a rattle can or Sherwin Williams FA3XEP, also in a rattle can. My process is to lightly scuff with scotchbright to give the primer something to latch onto, then wipe down with acetone on a paper towel until black stuff stops coming off on the towel.
That means bulkheads, j-channels, ribs etc get primer & the skins get a streak of primer about an inch wide on the interior surface where the rivet lines are and where there's a skin overlap.
My feeling is that if I get water trapped in there between the mating surfaces, the primer might give some measure of protection, but otherwise I'm not worried about it.
Hopefully it's obvious that I'm talking about stamped parts that are alclad. Anything that's not alclad should typically be primed or painted, and Van's is pretty good about calling that stuff out.
I get that I could go crazy with alodine and Akzo Nobel and have an airframe that would last 100 years, but I'm 54 years old, so an airframe that will last 50 years will still outlast me by a good bit.