HDMI was a mistake.
I just love how CEC keeps surprising me by unsolicitedly turning on devices and switching inputs on the HDMI chain between TV, projector, AV receiver and BD player.
It's also great that all of those "bidirectional 1<->2" HDMI switches aren't actually bi-directional in that they do not transmit the ARC signal back. It took me several weeks to find an overpriced obscure device that is actually able to send one video signal to two display devices while also sending ARC back to the AV receiver. HDMI is so easy to use.
It's amazing how progress has resulted in 15.4 AV Receivers, when all I want is an affordable (= less than 300€) 5.1. But they just stopped making those.
Apparently, there's a market for people who install 15 speakers plus 4 subwoofers in their living room. There are folks getting paid preparing movie audio tracks that cater to this audience. Who are these people and why do they watch "Gravity" every weekend?

It's fun how people recommend not to use HDMI cables longer than 7m for a reliable 4K signal. You can buy 20m cables, sure, but the spec doesn't guarantee that it'll work.

My dear, where am I supposed to put the projector and the AV receiver in the living room if there's a <10m cable limit between them?

(My current setup is 1080p and the looong cable works fine. But who knows if a future hardware upgrade will require changing the cables, too.)

@hzulla Bei einer 15.4-Anlage würde ich ernstlich überlegen, ob eine Beolab 50 oder gar 90 nicht die bessere Lösung wäre. Klanglich fast sicher, preislich vermutlich auch [langfristig].
@hzulla 2.1 is perfectly good for me. Anything more is just wasting money.
@hzulla I've got one of those movie buffs in my close friends circle... ¯\_(ツ)_/¯
@Moaske anyone who installs more surround speakers in their living room than I did is obviously insane. /s