For Morchella to revert to a reduced structure means that it evolved from an ancestor that did not have structural complexity, which of course means a yeast. If it would have evolved from a mold, it would have conidia instead of ascospores in addition to all of the other physiological characteristics of higher, filamentous fungi. Mycelium lacks extracellular enzymes. Residual autolysis is disadvantageous for mycelium. Intolerance of dehydration of mycelium is yeast-like.
Ascospores are disadvantageous.
Endotrophism was a baffling concept, when, in graduate school, I found the yeast Nadsonia fulvescens to be forming spores in distilled water. The general assumption was that yeasts required a nutrient medium to form spores. The authors said some bacteria were known to be endotrophic by the criterion that they formed spores in distilled water. Nothing more was said, and no specific references were given. In retrospect, all filamentous yeasts are probably endotrophic, but Nadsonia is not filamentous.