Since you obviously went to the iFixit site to get the image, you surely read that the plastic "bumper" is an audio channel for the mic, which also provides pressure equalization for the sealed phone, via those functional holes.
I will concede not knowing the purpose of the plastic piece outside of water resistance.
And what about Lightning headphones frequently "cutting out"? I've got a pair and they function perfectly. And what reliability issues with Bluetooth? Maybe you're using the wrong set of BT headphones. Have you tried a pair of W1s?
Perhaps I could own a better bluetooth headset to pair with the phone, however, my primary go to headphones have always been a wired set of in-ear monitors. Most consumer level bluetooth headset options I've demoed in-store appear to favor muddy bass over sound accuracy.
Discussion of Lightning headphone reliability can be found here: https://forums.macrumors.com/threads/lightning-headphones-not-always-working.2012535/
Discussion of bluetooth reliability issues with cars is discussed here: https://forums.macrumors.com/threads/iphone-7-and-bluetooth-car-audio.1997414/
The digital vs. analog audio was another argument towards the move to remove the jack - but doesn't a digital source does need to be converted to analog vibration to produce sound?
And there's no mixed message. Apple told us why they removed the headphone jack -- they needed more room. The MBP has more room. It's also used by people who don't use iPhones, so Apple can't control the experience as well and must service those who don't -- just like Beats headphones mostly still use micro-USB instead of Lightning for charging. There's also no real market for USB-C headphones yet. And Apple clearly wasn't prepared to add Lightning to the MacBook. If Apple sold USB-C headphones, it might be a different story, but that doesn't make a lot of sense to put two competitions audio standards on the table within a month of each other either, to the extent Apple even intends to support USB-C as an audio connector. Apple didn't put USB-C charging cables in the iPhone 7 box either, for good reason -- 200 million iPhone 7 customers over the next year vs. less than 15 million MBP over the next year -- in the meantime a lot of incompatibility and inconvenience for their iPhone customers who don't use MacBooks.
It seems to be a mixed message because Apple likes to promote device interaction within the Apple ecosystem. While many casual iPhone users may sync their devices to PCs (or in some cases, use them as a standalone device) - Apple likes to promote iPhone/iPad to Mac/MacBook ecosystem interaction to share messages, notes, calendar dates, photos, videos, etc. between Apple devices.
Syncing via iCloud and sharing via AirPlay is a solution to get away from the iDevice to wired tether, however, many consumers still connect their iDevices to their Macs to sync via iTunes. To connect the latest iPhone 7 to the high-end 2016 MacBooks, you would require a dongle, much like the headphone solution for the missing headphone jack.
I believe it is a mixed message because Apple talked about the need to remove the headphone jack to push the industry forward - then release their modern MacBook models with no standard USB jacks to directly connect the iPhone to the MacBook. And while the MacBook switch to USB-C is intended as a way towards future proofing, they also carry the infamous headphone jacks they declared obsolete just months prior. Apple should care little about causing a stink with PC inconvenience if they didn't care about causing inconvenience with headphones.
Since Apple insists on using lightning ports with their iDevices, one way they could've avoided these mixed signals would be to bundle the iPhone 7 with lightning to USB-C cable with a USB-C charging brick.
Thankfully, the "antiquated" 3.5mm headphone jack is still available in the MacBook models, as it would cause quite a stink for musicians who like to use their computers for recording and production purposes.