To paraphrase Andy Grove, "TMG is the heart and sole of Intel. You cannot understand Intel unless you understand TMG."
Intel being vertically integrated with manufacturing is an economic/investment decision by the BOD. It works out that semiconductor companies that can afford vertical manufacturing have it -- the rest go fabless. Intel has been able to afford it, and it having vertically integrated manufacturing provides many benefits - lower cost, but most importantly, control. Consider the situation where an emergency stepping/revision of a microprocessor needs to be released. With Intel owning TMG, the effort is "all hands on deck" to get the new reticles and resume fabrication as fast as possible. If TMG were an external company, and Intel wanted an emergency reticle replacement, sure the external company will do it -- for a price. That said, should Intel no longer be able to afford TMG, it would be setup and an independent company.
The other way Intel might split from TMG is as follows. The end of Moore's Law is reached. The last process is at 3 nm. Extensions such as 3D stacking of transistors and such could improve density over time, but it's costly and you'll always be battling a heat removal problem. So, Moore's Law is dead. Feynman famously said, "There is plenty of room at the bottom." Well, Moore's Law got us to the bottom, and now it's all filled up. In this case, Intel would also split from TMG -- as TMG no longer provides a competitive advantage. Since the technology is pretty much tapped out, Intel can buy it as a commodity from several manufacturers. No reason to keep the now low-margin TMG.
Had TD executed on schedule with 10 nm, followed by 7 nm ramping about now -- well, TMG would be closer to the end of Moore's Law. Watch out for someone who is qualified to be CEO of a major corporation be appointed as the head of TMG.
Taken from @VDBYZ4o-2kls, I hope the OP does not mind. Excellent read that needed to be on top.