No it's not opinion.
The easy part first. bugs and bird poop is acid and eats actually the paint off. It's important to remove it immediately. normally it's enough to spray it off with water- water is thinning the acid in the bird poop and insect bodies and dissolves it.
the problem is more the chitin from the insects. this glues with the guts on the paint. if you wipe it off, it will scratch the paint. this is what you see- scratches. that's where I said: layer newspaper on top and water it good and keep it moist- it will loosen the guts and soften the chitin and you can wipe it off easy without scratching the paint.
Salt is not marrying with your paint. salt sits on it and when it dries, it is crystals. if you wipe over it, you scratch the paint.
Salt removal is easy. water water water. and after spraying it off good, use like normal good car wash soap and a micro fiber rag. wash the rag out often and good. When I drove through salt, as soon as the weather gets better, I drive through a car wash with them soft rags instead of them hard brushes. I buy the underbody wash. this takes care of most of the underbody and sprays the salt out of the corners. you can do it by hand with the hose, but you will not reach all regions like the carwash.
then wash the vehicle at home like normal. that takes care of it.
and like I said. if you have corrosion, there was a damage before. other than that- there is no way the vehicle does rust.
Ok- I always spray underbody protection on my underbody and frame as soon as I bought it. this also helps to keep the underbody rust free.