One multi port adapter can take care of any connectivity need. It's actually much easier to have several things plugged into a multi port adapter and be able to just plug and unplug one thing vs several. Any legacy item can be connected to a newer MacBook through an adapter but when USB-C becomes the norm, there is no way to plug it into a machine that doesn't have C ports.
No doubt it was a business decision by Apple not to add a USB-C port along side older ports. However, it's not like you can't still use all older peripherals via an adapter. Best of both worlds.
Of course there are adaptors, but they are not growing for free in trees.
Apple has a baseline price of $30/30E for whatever adaptor they force you to use because they chose to not put a standard port.
Also, there have been reports of bad USB-C adaptors/cables that can fry your computer and/or peripherals.
What's the point of having a sleek computer with smooth borders when you have to carry a bunch of adaptors just in case?
Now it's getting slightly better, but if you do business and have to connect to projectors, you never know in advance if you will need the DP to VGA, DP to DVI, DP to HDMI, so you have to take all of them because they did not thought it was a good idea to make one with 3 outputs (well, it is not that they did not thought it thru, it is mostly that their eyes turned into cartoon $$$ and decided to make different adaptors to make you pay 3x)
We could argue that most people don't use these adaptors, but for USB?!
In the end we need to see where we are heading, to totally proprietary and closed laptops, as closed as iPads.
Look at it, the rMBP 2016 has the SSD soldered, it has nothing that you can exchange, not even the connector to read-out the data on the SSD is standard and you need to ask them to exfiltrate it for you shall the motherboard die.
It get's worse when you realise that they will most likely keep shipping the iPhone/iPad and other devices with standard USB, and that they won't drop lightning for USB-C (thing that would make sense, but then they would stop getting money from MFI chips)
Look at how previous transitions were done.
The Firewire port was slowly phased-out, a number of generations of laptops had both USB and Firewire. Transition went smoothly.
They could have done the same with USB and USB-C connectors, they just choose not to.
Anyway, it does not really matter.
They abandoned the cMP for the crappy nMP, and so they will continue becoming some empty (as opposed of creative) company making devices for watching cat videos.