Except the jobs could be leveled separately, or at only partial carryover, and people would still enjoy the idea of reasonably overlapping skills across reasonably overlapping aesthetic or skill-source.
Alternatively, one could go the other way, allowing all overlapping skillsets to overlap in abilities unlocked. If two forms of aetheric manipulation are nearly identical, should learning one have no consequence on learning the other?
Such is a matter of cohesion, of world-building and sense of character. Why try to reduce it to "You're lazy; gitgudgrinding" when it's far from the main point of the idea?
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Personally, I think we should return to actually having some manner of character customization in terms of skills learned, but as more than the half-assed Additional Skills menu we had originally, able to take synergetic traits from other classes previously learned in place of native skills or traits, as to customize our playstyle nearer to what we already know if we so wish. Ideally, such would almost never be optimal, but always near enough to so to find a place.
Alongside this, I'd allow jobs to again use any and all primary stats, albeit to varying degree (via soft caps at varying points consequent to their more standard or optimal builds), with each piece of gear that is just a reskin of another collapsed into a single immediately-dyable piece of gear. If I've played healers thus far and take up Monk, I'd have the option of running a sort of Monk capable of providing support, with a higher Mind cap to boot by which to better make use of my previous gear.
That's just me, and despite what it'd mean for inventory bloat, and ease of leveling in every way but shared experience, it'd likely be a very niche want for the game. But it has nothing to do with laziness, only a want for cohesion and more efficient systems.
He isn't the first or last to mention the idea, however, and certainly doesn't represent the entire group of people who like the concept (albeit for other reasons). When I first visited this thread, there were two more suggesting the concept right on the first page under New Posts.
That one person wants X for Y reason does not mean everyone who everyone else who wants X does so for reason Y. My own are much like the last dozen or so threads on the subject since early ARR: interest in character-building, world-building, and the cohesion possible within either or both.
As you've already pointed out, I've defended the subject, not the TC. My views are obviously quite distinct from his despite my likewise liking the idea of (a particular version of) shared classes.