the selectable tip is just a way to convey information to the power supply. A better way would be to keep the selection of tips we have now and put a tiny amount of smarts into new devices, so that they will talk (say) 5V signals to negotiate the power required, then use the same cable (and tip) to get that power. Of course, you'd still need manual override for dumb devices, and since this would cost the manufacturers money at both ends, it's unlikely to happen. Accessory power supplies are a lucrative sideline for laptop (etc) companies, and if they accept a generic supply suddenly there's competition where there never used to be.

Interestingly, some devices are very input-tolerant, especially anything that accepts alkaline batteries (which discharge from over 1.8V to under 1V). So you can power via the battery connectors using just about anything. Many digital cameras are like this - the "4.8V DC in" actually means 3-12V, with the limit often being your 15V CMOS one. I've run my Canon off a "fake battery" that had 8 AA NiMH cells in it... instead of 200 shots from the LiI battery I was getting 500+, but it was pushing 9.6V into a nominal 7.4V input.