Assuming for the moment that reboots were a serious concern, and not just a fabricated excuse... it's better for Apple's reputation for old phones to be slow than to be flaky.
With the former, people were assuming that Apple's shiny new OS required state-of-the-art hardware to run smoothly. It just appeared as if technology was advancing rapidly, and one had to buy the latest iPhone every year or two to keep up.
With the latter, there would be noone to blame but Apple, and they would develop a reputation for unreliable hardware, like Samsung or worse.
They still are. As a battery ages the internal resistance increases. This leads to brown-outs under high current. This isn’t unique to Apple, it’s just how batteries work.
Anything less is deceptive and anti-user.
The only thing they have changed is that they now have the OS level pop ups, the feature still exists.
Apple get phone sales from me because of this stance.