It’s really no different here. We’re talking about, basically, heated fruit sauce mixed into yogurt as a fast and easy breakfast. This isn’t a gourmet restaurant offering.

Frozen berries are a technological improvement on jam/preserves which dramatically reduces the amount of sugar you need for long-term storage, but requires some energy use for refrigeration. If you plan to heat them, mash them up, and stir them into yogurt, they’re pretty much interchangeable with fresh berries, but are cheaper, keep for months, and can be eaten in any season.

Or is it the microwave you are opposed to? If you prefer, feel free to heat the berries in a saucepan or bake them or whatever. I find a microwave to be a lot faster and more convenient for pretty much identical results, but suit yourself..