Well, Intel can still manufacture CPU's at TSMC if they wish so even with their own fabs. The problem that creates is perception of their competence if their own fabs can't produce the latest and greatest they design chips for, one may start questioning if their latest and greatest node design isn't all that good. Again, that would just be a perception to an outsider, not necessarily an inferior product or node design.
Well, sure … The other option would be if Intel would just go there and would buy into any other foundries in any greater volumes or even on a large scale (Pro-Tip: They are their
direct competitors). However, this would equal total
declaration of bankruptcy (including the most fatal loss of reputation, not just on the stock market ...). As a result, their stock price would
maybe not hit rock bottom, but they should make billions in losses in the low to mid double digits, to say the least.
The least what then would going to happen, will be, that they have had to hang up their foundry-business completely – as
no-one in this business is ever going to a) accept their lack of reliability or b) would trust
any of their promises in terms of fabbing chips, yields, schedules or similar in general ever again (which seems to be already the case, given that more and more partners jumping their ship lately and they have less and less partners which are willing to have their chips produced using Intel's nodes ...).
But hey, we're still talking about Intel. A semiconductor manufacturer and the most iconic chip producer in history, who consider themselves being the biggest chipmakers since the invention of electricity. Apart from that, there would be some rather real chances for massive Chinese industrial espionage, the most sacred of Intel's crown jewels: their state-of-the-art processors and elementary x86 IPs.
So, before something like that is going to happen, I don't know ... It'll be a cold day in hell when that happens.