Nothing is perfectly secure on its own. No system designed or checked by humans ever will be. After all, the Xbox One was indeed pwned, relatively recently. However, because the juice wasn't worth the squeeze for so long, it got pwned years after it was a relevant, money making console.

Novel jailbreaks for ancient iPhones are not worth much. But attention on current, brand new devices means increasing the danger that a mistake gets found, which increases the odds that that mistake is found by someone who wants to sell it for the most money. Also, from Apple's perspective a zero-day in the bootloader on macOS also means a zero-day in the bootloader in all of the billions of iOS devices out there too. They do not want to give anyone anymore reason than what already exists to try and pwn LLB or iBoot. Given a happy path, all of that hacker energy for "put Linux on my M1 Macbook" is put towards device drivers and support, rather than "how the hell do we get an alternative kernel booting on this thing".

Fewer bullets pinging off the armor. Fewer cracks in the fuselage forming. Fewer knives to dodge. All of it means Apple's boot process for their current devices are less likely to be pwned before they turn into e-waste (whenever that is, not making a comment on Apple's perhaps accelerated or otherwise practices in obsolescence).

Just like a jetliner will eventually succumb to entropy and become dangerous to fly, so too will a lot of "secure" software. You only need to actively maintain a jetliner while it is flying passengers or cargo. Once it is retired, it can rot, people can break into the husk of it at the junkyard and fornicate and smoke crack and smash windows and steal parts of the fuselage. At that point, who cares?