It's very simple, don't leave a bike that is 'valuable to you' anywhere. My commuter is over 30 years old, but still a valuable to me and reliable bit of transport - I don't leave that in anything other than a secure space. Many bikes left 'outside' work are quickly striped for parts. The police don't have enough resources to investigate this sort of crime.
I agree with your post, other than the first 3 words "It's very simple"....
It isn't always very simple. There have been times when as a solo tourer, I find myself in areas where I would rather not leave my bike unattended, i.e. any city you care to mention in the UK.. However, I do have to use the toilet now and again. As a result of other public service cutbacks, public toilets are not easy to find. So my solution usually involves a visit to a pub, or a large supermarket, or a bigger branch of M&S, or John Lewis if I am looking for a more luxurious (and usually clean!!) experience. Unfortunately this means locking the bike to something solid for maybe 15 minutes or so by the time you find the toilet within said shop, do your business, and return to the bike. Plenty time for someone with an angle grinder to have made off with your bike.
As a retired Police Officer, it pains me to say it, but I probably wouldn't even waste my time reporting any bike theft - unless insurance company insist on it (and I probably wouldn't even claim insurance by the time they take off excess and then increase premium it wouldn't be worth it). I know how much effort got put into bike theft 20 years ago (i.e. not a lot), and it has got worse; much worse since then. I think we just need to accept that as cyclists, bike theft is just an occupational hazard. That is why I won't spend much on a bike that I might have to leave unattended, even for that 15 minutes in M&S! I do have a couple of nicer bikes which as you say, I wouldn't let out of my sight, but that makes them a bit impractical for anything other than a few hours "there and back" rides from home - which isn't what cycling is about, for me.