I agree with some of what you say DIY BOB. Bathrooms and kitchens do get updated and many are every 5 years. Even although they get updated, the majority of the existing water supply pipework is re-used and new connections are made to it. Copper in my opinion will be there long after the last flexi in the installation.
I don't need to or use flexis on a btw toilet. A concealed cistern can just as easliy be piped up in copper.
Pumps are connected with flexis nowadays as pumps need servicing. I'ts easier to connect/disconnect a flexi going to a pump than it is to repeatedly disturb an olive on a compression fitting. High pressure pumps need a small degree of movement in them too, so that is one advantage of where a flexi is an adavantage. However the flexis are usually very high quality ones that are tested to be used at high pressures.
My original post is about these cheapo imported flexis that are now common place. Even taps come supplied with these poor quality hoses. I know, I've seen a burst one in PTS last week.
As to plastic plumbing, much of what is being said about it is purely conjecture. We are surmising that a brand of push fit will last or fail after a certain period of time as it's not been in service long enough to validate/invalidate any manufacturers claims about the longetivity of it's service life.
I like the tried and tested approach to what I do and in my opinion, copper ticks all my boxes.