That's a possibility, the new MBP could have a lighting port.
But what if Apple pushes for wireless technology? They put only one port on the 12'' Macbook, talking about their vision for a wireless machine. Why should they add lighting to that instead of a second USB-C port?
I'd rather have more USB ports on a Macbook or Macbook pro than having a lighting just to connect my headphones (using an adapter)?
Besides, why remove the 3.5 jack from a Mac? They can't make it thinner, as long as USB-C is there.
I'm not a big fan of wireless headphones, it is just another thing to charge. But I'm not a fan of dongles either, and bluetooth is standard so if I really have to make a choice I prefer bluetooth.
Beats have lots of bluetooth products to offer, my guess is they'll ship a bluetooth version of EarPods, with a quality comparable to their wired version, and hope many customers will buy more expensive bluetooth headphones.
To those willing to use their 3.5 headphones Apple will sell an (expensive, of course) adapter.
More money for them, Tim is happy.
I'm sure Apple is going to push for wireless technology. That is a given.
But to address your points, the 3.5mm Jack opening is much thicker than USB-C. However, I've never seen the edge thinness as the main reason motivating any of this. On the phones, it's the volume of the total hardware, most of which resides inside the phone which is most definitely constrained for space to add new features no matter how thick or thin the iPhone is. The opening is like the tip of the iceberg. The MacBooks are something different as the real estate along the sides is limited. In the case of the rMB, they have very limited space on each side of the rear case as nothing can seemingly go underneath the keyboard, least of all the 3.5mm Jack.
The reason for adding Lightning to the rMB instead of a second USB-C port is simple -- it's for compatibility with headphones purchased exclusively for use with the biggest selling, most profitable product they sell. Imagine a new iPhone customer buys new Lightning headphones, and a new rMB, and is told he has to use an adapter in order to switch back and forth between the new MacBook? Even if the customer buys a wireless pair, they will still come with a Lightning cable for charging (which they will likely do on the MacBook), and to listen when the wireless battery runs out -- and it will routinely for all customers.
Add to that Apple's marketing on the rMB -- they weren't going to include any ports because they see it as a wireless solution, but they needed a port for charging, so they decided to make it USB-C. Now that's total BS, right up there with they'll never make a 7" iPad because you'd need sandpaper to sand your fingers down to use it. The reality is they had two ports -- the second was a headphone jack, which they were perfectly capable of eliminating in favor of BT. Just like they were never going to release a Mac without a data port, or an iPhone without one either -- a device more than capable of going completely wireless with inductive charging which Samsung is already effectively using.
By replacing the headphone jack with Lightning, Apple is adding convenience to their core customer base, while also giving in to those who refuse to embrace Appe's wireless philosophy, because it's not a 2nd USB-C port, it's a headphone jack, that happens to be a decent data port -- just like the USB-C is first and foremost a charging port, with data thrown in for good measure. That way they still encourage wireless, since most any data use will require adapters (just like it does now with USB-C, and likely for a couple more years). The thinking here is that the customer will most likely carry only a Lightning cable to charge all their devices, thus leaving the USB-c always free for data (unless they rely on Lightning headphones).
Lightning also propels the merger of iOS and macOS, and consider Apple's desktop peripherals are all moving toward Lightning as well.
Now, I have no doubt Beats will have USB-C ports, and Lightning ports since they serve a broader market than just Apple. A standard set of Beats wired or wireless headphones will likely have a USBc port on one side, and Lightning on the other. This lets them be charged by Lightning cables, or connected to an iPhone or MacBook with the same cable in reverse. Also, it allows pass through charging and listening at the same time. It also allows friends to daisy-chain off your ports to listen along with you -- something some Beats headphones already supports.
Unfortunately, if Apple makes USB-C earbuds, they'd be doing it strictly for their MacBooks. And a loyal Apple customer will still need an adapter to use them with their iPhone and iPad, or have a separate set of Lightning headphones. Not an ideal situation for someone that just spent a few thousand on a new iPhone, headphones, and MacBook.
If Apple removes the headphone jack on the iPhone, it's hard to imagine they'd remove it on all of their products immediately, since most iPads and Macs have plenty of room to accomodate not just the opening, but the internal space as well. Add to that Macs and iPads are used by customers who don't use the iPhone. While the rMB is totally geared toward Apple's wireless services, like AirDrop and AirPlay, and makes sense it will be a part of an Apple eco-system which includes the iPhone, rather than a broader market, it doesn't make as much sense to drop the headphone jack on all others, but rather add a Lightning port for Apple customers to use.