Any explanation you gave is, with all due respect, wrong. Part of evolution is the process whereby beneficial mutations are passed from one generation to the next. To say that one set of mutations can occur in one genus but categorically can never occur in another shows a drastic misunderstanding in how evolution works.
Let's see, where shall I start?
Ah, there: by that logic, one could even say that it would also be perfectly plausible to say that snakes and lizards could develop functional gills, sharks or coelacanths (those who actually know a thing or two about the unusual anatomy of the coelacanth know what I'm talking about) could develop functional lungs, insects developing an internal skeleton and perhaps even a fully developed bloodstream.
However, the evolution of species is
limited by the anatomy and physiology of the species on the matter, and a development that is only a few mutations away for species A could very well be next to impossible for species B, just because species B has a different anatomy that may even make these mutations and adaptations impossible because the anatomical structures that are improved and adapted in species A could be so different in species B that evolving and adapting them in the same way as species A did, would simply be impossible.
And before you're going to ask; yes, I am very familiar with the concept of converging evolution, i.e. unrelated species (or more correctly: lineages) that develop similar adaptations (and often a somewhat similar appearance) because they adapt to the same circumstances.
However, I would also like to point out that, out of all things, converging evolution in species often shows how the basic anatomical and overall genetic characteristics still limit the extent to which these animals can adapt, and even then, the basic design of the two converging species already has to be quite similar
if they are ever going to evolve into two clearly similar species.
And if the two converging species are
not quite similar to begin with, then some degree of converging evolution may still be possible, but it would only be more likely that the evolving species on the matter evolves adaptations that still fulfill roughly the same purpose as the adaptations of the species it is "mimicking", but these adaptations would propably still be very different solutions to the same problem and would propably be unique to the species on the matter.
And after all that comes the issue of propability: it may not be
impossible for a species or lineage to develop certain features or adaptations, but that doesn't mean that many of the more unlikely evolutionairy possebilities are statistically
likely to happen, because the greater and more fundamental an adaptation is, the more factors must support or stimulate the development of such an adaptation, and don't forget that huge changes also require the development of several intermediary stages, and it is not uncommon that animals evolve into such "intermediary stages" for entirely different reasons.
For example, lungs developed from primitive swim bladders that were
originally hydrostatic organs, and the fact that some types of swim bladders could actually double as an organ that could extract oxygen from the air was in fact little more then an unforeseen side-effect.
And the more radical a change, the more supporting or stimulating factors are needed to make a species or lineage evolve so much, and the more unlikely it becomes that the species on the matter actually
makes this radical change.
So in the end, it is more likely that different species or lineages develop their own *usually* unique solutions to the same problems, rather than mimicking the exact same solutions that work for other, completely different species and lineages.
To extrapolate this to the claims that some dinosaurs could develop brains that are similar to those of mammals at some point: I'm not even excluding that dinosaurs could develop complex brains, and I admit that seeing the great degree of intelligence that certain cephalopods, crows and parrots have displayed surely forced me to re-evaluate my previous opinion about the "unmatched" qualities of the brain design of placental mammals.
However, I stick with my point that, even though some branches of dinosaurs would have been quite capable of developing complex and relatively advanced brains, these brains would most propably not resemble those of mammals in their basic design, i.e. the 'wrinkly' structure. Furthermore, the brains of the most intelligent dinosaurs were developing more like the brains of birds than the brains of mammals; the capacity of the brain was increased because dinosaurs and birds were developing a thicker cerebral cortex, instead of a 'wrinkly' structure that enlarges the surface of the cerebral cortex.
Had the most intelligent dinosaurs developed in even more intelligent creatures, then they would most propably develop a cerebral cortex that only becomes thicker and larger, instead of becoming wrinkled like the brain of a placental mammal.