"simple mutations which benefit survival" would have to start out as a single microscopic mutation of a cell. Then, continue mutating in a fashion that would eventually be a benefit - a "happy accident".
It seems to me that a mutation within a given species which initially serves no purpose - would disappear.
Are suggesting that "happy accidents" are the primary driver of most of evolutionary gains?
No but a bottom dwelling swamp fish might get fins that, over a long time, increasingly help it to maneuver on the bottom by becoming more leg like. In fact, that’s what the fossil record shows. Further, the organism might find that going further toward the land is helpful in avoiding predators and over time you get amphibians. I’m oversimplifying but that’s what fossils show happened. With very few land animals, laying eggs on shore would have been an advantage in survival. Of course this happened over a long time. Some came in rather quick spurts but most over millions of years. Having a new land environment and plenty of oxygen brought quick proliferation of species.
Forgot to mention the Cambrian explosion where in the oceans, there was a terrific rush of evolution following the atmospheric and temperature changes which followed snowball earth. Over 50 million years many of the ancestors of modern creatures evolved into being. I guess you might call it happy accidents but to me it doesn’t seem accidental as you say advantageous mutations are retained and others either can be carried forward is it doesn’t disadvantage the creature or is is a problem, it disappears over generations, usually one where the creature doesn’t pass it on.