> It takes just 5 years on permit C
This is accurate, however, you need to spend at least 5 years in Switzerland to apply for a C permit and have been resident a minimum of 10 years in Switzerland in total, including the last 5 continuously. You'll also need to meet minimum cantonal and communal residency requirements, so if you move canton or even communue at 9 years 11 months, you'll probably have to wait another few years. The process itself will take a few years also.
After all that, you are still defined as a "US Person" for tax purposes and thus banks are required to disclose your information to the US. US Persons are any person with tax liability to the US. You're tax-liable as a citizen (also as a green card holder or possibly a married partner of a US citizen). So having spend 12-13 years minimum in Switzerland to get a passport, you then have to give up your US passport, which will take time and is also permanent.
Usually it doesn't take 12-13 years minimum with stringent residency requirements and the requirement to learn a foreign language to open a bank account ;)
All that said, it is my understanding that the PostFinance-only thing was a reaction to the Credit Suisse fine from 2014. After that, Swiss banks closed accounts and mortgages of Americans without warning as a way to avoid any future risk - a bit extreme, but they were in a legal pickle: Swiss law prohibits them from disclosing account information to anyone not at the bank and includes custodial sentences for the disclosing employee. There was thus no mechanism for disclosure to the US that was actually (at the time) legal in Switzerland. This has been remedied. Banking secrecy still exists, but banks are allowed to transfer information to the Confederation (Swiss Federal Government) and the Confederation can of course then transmit it onward to the relevant parties.
Even more bizarrely, the whole FATCA issue was mostly about wealthy US residents using offshore tax havens to avoid taxes, not your average expat who simply wants a bank account.
I believe that US expats can now open accounts at Swiss banks other than PostFinance again, although I caveat that statement by saying I'm not a US person.