The Builder - it's interesting how sizes change over the years. A few months ago my power-shower pump got noisy so I decided to change it before it gave up on me. I installed the shower many years ago with 18mm fittings; the replacement pump, same make and model, needed 15mm fittings.
I assume all these changes to smaller pipe sizes are attempts to reduce the costs but it must be trivial on the cost of a house.
I do understand your point about the feed to the hot-water cylinder; since all my bathroom taps etc were indirect when the house was built it wasn't a problem for me but I see the need to be careful if modifying a newer system.