6.0 Red Mage speculation

[Archwizard]

If you increase mana generation, you're part of the way to making the spender phase superfluous

I fail to see how it would do that, in part because our Mana generation has already increased this last expansion thanks to Scorch and Enhanced Manafication, and based on the rest of our leveling experience, I expect that will continue to be the general upward trend for us in the future.

If the spender phase maintains its position as the capstone of our damage, then increased Mana generation doesn't necessarily diminish the phase itself, just makes it more frequent. Like if BLM somehow got a way to speed up Polyglot charges, for instance... or like any melee job's resource gauges.
All this depending on how they choose to go about increasing Mana generation of course (new spells that give above-average amounts of one type of Mana or some of each type, increasing Ver-proc rates, adding a small amount of Mana to each of our oGCDs, an Anatman-style skill or adding a small amount of Mana to Cure in-combat for downtime, etc), but if anything it would just reduce our necessary time spent slinging ranged attacks.

Your critique comes off as doomsaying more than logical concern.

You could do what I do, which is stand in melee range and just use it on cooldown.

Like I was saying, all you're essentially doing is mashing another free swing into the combo we already have (since its timer roughly lines up with it anyway and it's affected by Manafication). I'm not sure that tangibly makes a positive difference in terms of our gameplay, at least no more than tacking another capstone onto the end of our combo.
I'll grant you could say the same thing about any of our other oGCDs, but you don't need to be in the melee to use them on cooldown, they're not synchronized with our melee phase, and their actual role is to help fill the frequent gaps between our interchangeable dualcasts so we don't feel severe downtime; their asynchronous nature also gives them plenty of room for expansion, like shrinking their CDs without worrying about how they line up. By contrast, our melee phase hardly has any downtime to shrink.

Right now, Engagement exists not because the devs felt we needed another melee skill, but because they wanted to give us a consolation for occasionally being unable to Displacement. Without that competitive aspect, the only justification for Engagement in its current form to continue to take hotbar space is for a sliver of raw potency (Physical damage at that), which wouldn't be strong enough for another job to consider worth keeping. It's exactly the kind of "superfluous" melee skill you were just warning against.
Without some other utilitarian element, once we get to the point the devs need to start trading out skills for us, it's at the top of the list of skills to chop, not build off.