... they are in perfect mechanical and cosmetic condition, with low mileage (21K on MB & 43K on Z4), having been garaged their entire life and maintained to perfection.

They both have the manual brake lever between the seats.

My other 2 passenger vehicles have electronically-deployed parking brakes. I don't know if they could be activated while the vehicle is in motion, and, although I lean towards curiosity, I'm smart enough not to put it to the test. I suppose in a doomsday predicament, it might be worth a try ... if it locked the rear wheels, it seems the vehicle might still be steerable. The alternative would be to try to shift into reverse while stomping the accelerator pedal to the metal (notice, being a technical perfectionist like you, I didn't say "gas", in deference to EV-o-philes).

And, now, for the rest of the story: Modern vehicles supposedly have independent braking systems between the front and rear wheels, thus the redundancy of a manual rear brake level may not be necessary.

Lack of a manual emergency brake in modern vehicles is very low on my list of worries. Here's what's higher on my list of concerns:

1. Crash-avoidance system which slams on the brakes in anticipation of a frontal crash ... what if it falsely deploys spontaneously in 70-80 MPH dense traffic, and you get seriously rear-ended, resulting in a multi-vehicle pileup? Low probability, but entirely possible.

2. Some of the crash-avoidance systems may attempt to steer away from a frontal crash ... what if you are in a situation where there is no safe alternative path that would avoid a full frontal impact? Probability not apparent to me.

3. Lane-departure warning system which nudges the steering wheel and will actually steer the vehicle around a curve with no-hands, then scream at you to grab the steering wheel. What if it spontaneously decides to aggressively nudge in the wrong direction just in time to incur a crash? Probably not very likely.

I have owned a wide variety of vehicles (too many, actually), including Chevys (3), Pontiacs (2), Buicks (3), Cadillacs (4), Corvettes (3), Mustangs (4), Lincolns (2), Austin Healy (1), Peugeot(1), BMWs (6) and Mercedes Benz (4).

BMWs have had the most problems: leaks, power window failures, power door lock failures, brake problems, electronic system problems. Even my trusty '03 Z4 had a problem with its ill-designed electric-powered steering assist and convertible roof leak. My BMW dealer did come to my rescue even out of warranty and replace the faulty items because of all the vehicles I have bought from them and other business I have pushed their way.

My strangest relationship has been with Cadillacs. My first was a 1977 Sedan DeVille, a gorgeous vehicle in its day, but an absolute slug in terms of performance. A huge 7 liter V8, but only rated at 195 HP. I swore I would never buy another Cadillac. Then came 2009 and the advent of the greatly-cosmetically-enhanced CTS (the prior version was a "trainwreck) "Batmobile". Awesome looks, 305 HP ... irresistible. About 4 years into ownership, it developed leaks. After several free attempts to repair, the dealer offered a sweet deal to swap for a 2013 model. All was well until mid 2018, when the paint developed cosmetic problems. Again, the dealer made me a "godfather" deal on a 2018 XT5, which I still have. It has operated flawlessly in every way for almost 7 years and 50,000 miles. All I have had to do is routine maintenance, most of which has been free or at extremely low cost, all by the original dealer. My only real expense has been one set of new tires. The mileage economy is fantastic: 33 MPG at 60 MPH on Natchez Trace Parkway (Nashville to Tupelo, 178 miles). 29-30 MPG at 70-75 MPH on interstate. I have zero incentive to trade it for a new one any time soon because the new ones look almost identical. I would rather spend the money on vacations.