Wasn't ever an issue for me. Same seemed to be the case for plenty of other rMB12 owners. Maybe it might be an issue for you and your needs, and sure there were lots of folks who posted about it on MR (most never seemed to actual own one though
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I'm not going to sit here and say that any one inconvenience on any Mac is a universal inconvenience (for all users). Judging by how many 12" Retina MacBook fanboys are on this forum at any given time, it's clearly not a universal inconvenience. But it certainly was enough of one to cause the machine to be replaced with the 2018-current MacBook Air (as, again, the 2-port 13" MacBook Pro is really the successor to the 2010-2017 Air, at least as far as Intel versions are concerned).
The fact that there are fans of something out there doesn't negate a majority rejection, let alone a rejection by Apple.
As for space - if it were me I'd drop the headphone port.
You'd have the vast majority of Mac users rejecting that. Most still aren't fond of the lack of a headphone port in USB-C iPads and post-6s iPhones. "Courage" doesn't negate that. Nor do expensive AirPods.
IMHO the rMB12 was pretty clearly positioned towards either very light needs users or as a secondary Mac for those wishing something with high portability while retaining macOS. I would envision any replacement as being similarly positioned.
It may have been positioned that way (and, for the record, I agree that it was positioned that way. But it was never PRICED that way. The Airs (including the 2018-2020 ones that directly replaced the 12" Retina MacBook, were priced more affordably. For a 12" MacBook to succeed, Apple needs to introduce it at a LOWER price-point than the Airs fetch currently. Otherwise, it's a sacrifice.
As for "complex computing tasks" -- not really sure what that means to you - perhaps you need a lot of screen real estate for what you do? I had no issues with checking/tagging/cataloging photos on a 12" display, web browsing, email, document editing, etc. Not as nice as a 13.3" display or a QHD external of course, but quite serviceable.
I'm talking pretty much any task that would be cumbersome to accomplish on an iPad Pro, but not on any Mac. File management tasks, disk management tasks (which, again, are made all the more annoying with only one port [hell, two ports is STILL pretty annoying, unless a Thunderbolt 3/USB-C dock is used in one's arsenal]). Not any specific type of workflow. Just things for which iPadOS still isn't ideal for, but macOS still is.
I don't know that anything has to happen to it.
Apple's been quite happy to have a lower-tier two-port MBP13 sitting alongside a higher-tier four-port MBP. I'd anticipate a higher level ARM CPU (M1X as some speculate) along side higher base RAM and ability to expand to 32GB RAM).
Yes there'd be overlap. Hasn't seemed to bother Apple previously.
It just seems needlessly complicated and likely to cause consumers not paying enough attention to steer the wrong way. Like we were discussing in the other thread about purchasing between a 2020 Intel 2-port 13" Pro versus the 4-port model. WE know that difference; but the average non-computer-savvy Mac person headed in that direction will look at the price first and not realize that the higher-two end models are different Macs.
I do like the idea of 2-port 13" Pro with M1 + 4-port 14" Pro with M1X + 4-port 16" Pro with M1X. That at least differentiates things in ways that are consumer-obvious. It also makes the "Pro" moniker on the 13" stupid again, but if the lower-end SoC still remains as capable, it won't be as egregious as it was during the Intel era.