How many times does it need to be said we aren't actually sure how long it would take until we actually do it? Seriously, people thought that just looking into making TWs would take a lot longer than it did with it only turning out to be 12 years in game which is just over 2 turns.
1) For gods sake would people using the argument that other polities are going to research something!? That is a rage inducing argument since people keep pointing out that it definitely applies to every single thing we can research and by that logic we shouldn't bother with anything not native with Avernus. There are a number of reasons that the other polities wouldn't want to research something since they may just want to do something else A good example is how our psykes are geared a lot more towards combat as a whole while the other polities are more support in general with most of our stuff being incompatible with how they do things. Also considering that Maximus has been researching animals for decades and still isn't even in the 40s don't think wildlife is as likely to work as working on something as complex to gain stats.
a) No, the estimates were maybe and it wasn't for anything but IIRC to fix everything.
2) And you keep ignoring the fact that the TWs are actually superior to SMs in a few areas to the point that a few are able to easily kill several when outnumbered with low quality gear relative to SMs. They just each have their own benefits. Fixing their issues would still make them better than SMs in a few areas. The areas SMs have over TWs are longevity, Warp resistance, reflexes and reproduction. Fixing the reflex and longevity issue would just leave the resistand and reproduction advantage.
3) And we can still do that once we are done. Why is that so hard to get.
4) No, we aren't totally sure until we do the research and as I mentioned it may not take as long as people think like with how people assumed that just researching TWs would take longer. And as I pointed out we can just not fix the life expentancy issue to save time.
5) And in canon TWs can be worth more than SMs even if not in general. Fixing their issues would make them even more valuable.
6) No, they are inferior in a few areas and have better strengths in others and as I keep pointing out that improving them would increase their value and put them closer to SMs.
7) He had the freaking Trueborn which can only be made from materials from the Primarchs themselves. Note, that TWs are noted to be physically as powerful as TBs.
It's pretty obvious that you are completely ignoring all the positives