This can be resolved any time cities want to build a lot more housing: https://www.theatlantic.com/ideas/archive/2021/01/anti-growt.... Outside of Tokyo: https://news.ycombinator.com/item?id=16704501, no or very few cities in the Industrialized world have chosen to simply build lots of housing, which will tend to bring prices down towards the cost of construction.
Similarly for Zurich, there are significant hills around and most flat space is already built up.
There are cheap houses available outside those areas, but getting a loan for them can be tricky since their value is too low and there's no money to be had from lending the money.
Furthermore, most people have their homes as security (not sure if it's the right word in English) for their loans. If the property value drops then many will be forced out of their homes. Also a lot of landlords and real-estate agents, will be out of a job, but that's probably a smaller issue.
I haven't looked it up, but from what I've been told a lot of it also has to do with investment laws for Pillar 2 and 3 retirement accounts (a certain percentage has to go into Swiss assets). Taking 1-1.5% over LIBOR/SARON or whatever the SNB rate is makes sense, but the fall-out in a couple of years if and when rates start to move up again isn't going to be pretty.
Owners generally keep a mortgage on their property (~65% of the property value) to off-set wealth and property taxes. Residences are taxed by their estimated rental values, which is treated as income. Interest payments are tax deductible, while the principal on the mortgage lowers the amount owned for wealth taxes.
The other issue is that most primary mortgages don't allow for early repayment, meaning that the principal either has to be payed off in full or refinanced once the payment period is over. Owners become conditioned to continuously roll over the principal on their primary mortgage.
To answer your question, because the expectation is that residential property will be mortgaged through out the entire ownership period, banks have to make sure that someone who can afford the payment on a 1% mortgage can also afford the payment on a mortgage refinance if and when rates eventually go up.
So if you buy a 1.25m house and take a 1m mortgage 5% is of that is 50k, so your salary has to be 150k.
Point 3) is wrong, they make still a lot of money with loans, a lot of people still take 10 year loans for somewhere between 0.6% and 1%.