Terminator armour should not be available to Primaris units. Its rarer than the Emperors tears and chapters have gone nuts to retrieve even just a single suit from space hulks etc because they cant replicate the tech. If Cawl manages to hand wavium new suits then GW are just pissing over their lore, again.
Terminator armour hasn't been irreplaceable for a long while in lore. They still produce it, it's just not cheap or easy.
Sources include Carcharodons: Outer Dark (where we see ten brand new suits still unpainted off the production line), and the various modern marine codices

So it's entirely possible for Cawl to introduce his own variant(s) for primaris users. What form that might take, we don't know.
Like I said, weeweeing over the lore. Terminator armour was special, a relic from the great crusade, it meant something to be issued a suit. Now its 'wait 6 to months and we will deliver it to you door'. Now another suit of heavy armour designed for primaris would be fine.
Also, if everybody becomes primaris or all old marines die out, what do the chapters do with the suits of Terminator armour? They wont fit anybody, they cant recycle because of the relic'ness of the suits,just stick them in the back of the wardrobe and forget about them??
Think of it like this: It is canon that the Deathwing is ENTIRELY outfitted in various marks of Terminator armour. It is CANON that Corvex (Sammael's bike) has been declared lost/destroyed numerous times, yet he always has it when he's in battle.
If any Chapter has the STCs for lost technology and has been holding onto it and not letting people know, it's the
Dark Angels.
So if they needed to, they could probably use the STCs that they VERY LIKELY HAVE and modify them to work for Primaris.
Or like Apologist said, just cannibalize the suits they have if they need to make ones fitted for Primaris Marines.
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Edited by PeteySödes, 03 September 2019 - 03:05 PM.
OT/Non-constructive