I. If such changes were possible in reproductive system of birds I see no reason why such changes in reproductive system would not have been possible in mammalian lineages too. It is only speculation that it was somehow impossible.
That seems to presuppose that evolution is purpose oriented, and will seek the best solution possible. I don't know of any support for such a view of evolution. Surely evolution is opportunistic, and takes a path that is good enough and that happens to be possible given the available variation within the genome (including variation provided by new mutations). And if it is opportunistic, there is no basis for expecting that it will find the best possible solution.
Now you consider sperms seeking lower temperature to be the source of their descent.
Sorry, but the idea of "sperms seeking lower temperature" is silly. Please don't ascribe that idea to me.
The opposite view - their lower temperature as adatation on environment is also possible.
I don't rule that out. Science answers the "how" questions, not the "why" questions.
To me, the traditional explanation looks like a reasonable hypothesis. But it is only a hypothesis. I don't have a problem acknowledging that we don't really know.
We have two poles.
You have not been clear on what are these two poles.
Let's end the political smears
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