Intel's struggle has nothing to do with is processors' design. Sunny Cove and Willow Cove ( aka Icelake and TigerLake ) were close to design complete before Jim Keller joined. Intel's problem is with their manufacturing, both technical and economical. And Jim Keller is not a Fab guy, nothing he could do to fix this.
Either it isn't worth it (too much coupling between fab and designs - not the right abstractions), or institutional inertia stops them doing it.
It wouldn't surprise me if the reason Keller left is because he wanted to outsource manufacturing to a competitive third party fab, and got denied.
Keller is an a computer architect, but architecture doesn't matter if you can't physically build the thing.
And yes, if for some reason Intel would want to fab their chips at some merchant foundry (see my sibling answer to yours why I think that's unlikely, but just for the sake of argument), I'm quite sure they couldn't just email the RTL's to the foundry and get chips back. It would take a lot of work to adapt the chips to the merchant foundry's process.
It is more of a cost/ economical issues, which jabl above provides a decent explanation.
So if they would fab their chips somewhere else, they would be sitting on a huge expensive asset producing nothing. If they couldn't find a productive use for their fabs it would likely mean the end of the company. And if they can't produce their own chips in their own fabs, why would anyone else want to use Intel fabs?
Further, the vertical integration of fab process and chip design is something Intel regards as a competitive advantage. For a long time this was very much true, but it seems the hard work by TSMC and others have made it possible to make top-end chip designs on a merchant foundry process nowadays.
Intel at some point tried to play the merchant foundry game, but it seems it wasn't successful and they shut it down. Which perhaps isn't that unsurprising, considering TSMC, and to a lesser extent Globalfoundries, have been at that game for decades and they're good at it.
So all in all, I don't think fabbing their chips at some third party is a viable approach for Intel. Either they fix their process or they go under. "Go under" not necessarily meaning bankruptcy, it could also mean a massively, hugely downsized company doing chip designs to be fabbed at some third party. I think they're still far away from such a drastic step.
Intel Management Engine and SGX on the other hand, are basically user-hostile parts of the hardware, with some bugs mixed in.
"Everybody" who has some knowledge in security (espcially with respect to side channels) knew from beginning that these CPU features were a ticking time bomb in terms of potential side channels.
What was unclear was how this (at this time played down by CPU vendors) potential threat could be used to create real attacks.
Going from potential threat (that "everybody" knew about) to real attack is the central achievement of the authors of the Spectre and Meltdown attacks (and their successors).
For a concrete blog post, see https://cyber.wtf/2017/07/28/negative-result-reading-kernel-...
Note that according to https://en.wikipedia.org/w/index.php?title=Spectre_(security... Spectre was published January 2018, i.e. this blog post is indeed older.
As I wrote: It was unclear how this potential threat could be used to create real attacks.
There's a video on AdoredTV about the topic: https://www.youtube.com/watch?v=agxSclh27uo