Re adapters, like thunderbolt to displayport / hdmi / vga / network, take your pick of soon to be redundant and need to replaced or plug adapter into adapter if Apple go fully usb-c.
Re lightening cable, why add a new lightening socket to a.laptop when it takes up the same space as a usb-c socket which is superior. Apple will just give a usb-c to lightening cable in newer phones and expect older iPhone 6 / iPad users to get a new cable if the need one.
Lightening cable would take far too long to charge a laptop anyway.
Apples and oranges. Apple owns Lightning, and has committed to it, and if the rumor is true, a lot is riding on it with the removal of the headphone jack -- I say again, they are not going to force new iPhone users to use an adapter on their new Macs just to listen to music.
Apple is no doubt going full USB-C, as will many PC makers competing in the ultra-portable space. It is hands down the best technology ever for such a device, and I go all the way back to the PowerBook Duo days, when my laptop had one native serial port and needed a dock for everything else.
Apple loses nothing by dropping HDMI, VGA, Display Port, or Ethernet. They don't make any products that otherwise use those connections, and those who presently use Thunderbolt pale by comparison to the number of iPhone users who use headphones between their phones and Macs.
So Lightning would be added primarily as a convenience for Apple's own audio products. With the rumored MBP, it leaves all four USB-C ports free for peripherals, since it can be used to charge the MBP using the same Lightning cable that iPhone users are already going to be traveling with. USB-C cables won't be easy to come by for a couple of years if you forget yours at home. The rMB already has a dedicated 3.5mm jack that does nothing but pass analogue audio signals, so what difference does it make if Apple replaces it with a Lightning port, since that is going to be the new Apple audio connector for its most widely sold product? Not only does Lightning leave the USB-C port free when used as a charging port, but allows it to optionally be used as a data port should an rMB need to attach two peripherals. Add to that anyone who has a substantial investment in Lightning peripherals will be able to use them with the Lightning equipped MacBooks now -- something that also saves Apple money for low bandwidth adapters for which they only need manufacture one. It's win win for everyone.