This is a good point and a good question, and we can only speculate on the answer. However, what I meant to say was that I am expecting MCDs to not be worth nearly as much as they are right now even if they are still needed. I am expecting MCDs to be obtainable by desynthing new items, especially items obtainable via the Scrip system. SE is big on catch-up mechanics, and they've stated many times that they're trying to make crafting and gathering more accessible. I do not expect anything that's needed for progression to endgame crafting in 3.0 to require the endgame crafting items from 2.x. Or, if they do, it'll be much easier to obtain the final items.
In other words, they won't raise the cost of entry, that would go against their goals with crafting/gathering in the expansion. Doing nothing raises the cost as more people enter the playing field trying to get their crafters/gatherers to 60. So, instead, they're going to lower the cost of entry, either by making MCDs easier to obtain or not requiring MCDs at all for endgame stuff, just like they did with FC3s. They can make MCDs easier to obtain by having them obtainable from new, easier to obtain items (like with FC3s, which are obtainable from vendor-purchasable items now), or by increasing the yield on current methods and/or increasing the range of items that can be used (like making the i70 non-head artisan's gear desynthable), or by reducing the number of MCDs needed from 3 to 1 for Supras (like they did with the token items to get Artisan's/Forager's mainhands), or by eliminating MCDs entirely from the Supra requirements (unlikely).
What I really expect to happen though is for Lucis to be either only slightly better than endgame level 60 3.0 gear that does not require Lucis to get, or Lucis to be inferior to endgame level 60 3.0 gear that does not require Lucis to get. Either way, I do not expect Lucis to be required for endgame 3.0 content, and definitely not for endgame 3.2 content. I could of course be wrong, but that really isn't the way they're going.
And, on top of all that, they've said they will be introducing new "crafting/gathering tomestones" (Red/Blue Scrips), which will be tradeable for endgame gear. I expect that gear to include main hands, just like it does for DoW/DoM classes. Having a Lucis will certainly make progression to endgame crafting easier, especially since you can then afford to hold off on upgrading your main hand for a while and focus on the rest of your gear. But I don't expect it to be required.
What I'm hoping for, and this is just a hope, is that Lucis items will be trade-in-able for credit toward the new endgame main hand, just like Zodiac Zetas will be somehow trade-in-able for a jumpstart of some kind toward the new 3.0 relic weapon. Chances of that aren't great though - crafting/gathering "relics" have so far been handled totally differently than DoW/DoM relics.
As an aside, while on this topic, I'm pretty sure it's been confirmed that endgame crafter/gatherer gear obtained via Scrips will not be meldable. They've stated a desire to get away from the meld-heavy endgame for crafters and gatherers due to the cost-of-entry issues it creates. So, endgame crafting/gathering is likely to be much more reachable than it currently is across the board.