A lot of the old gems will actually be worse for newer players once the patch is launched, imagine trying to full gem something with old gems (they still have rng in 4.0) and on top of that the 4th, 5th, 6th, 7th gem slot taking anywhere from 500-2000 gold each attempt (plus still having a chance to fail).
Even if you keep old gems or remove them, 4.0 is not a good patch for a new, lower geared, or even average player, no matter how you look at it. 4.0 is definitely a "rich get richer" patch.
- If we keep old gems in the 4.0 patch, people will still have to deal with the RNG associated with them, on top of the additional gold cost to socket, which makes it harder to gear than it is now for new/undergeared players
- If we remove the ability to craft old gems but keep the existing ones in the 4.0 patch, there will be a finite supply of the old gems which will make them harder to obtain for not only newer/undegeared players but also for middle of the pack average players. Wealthy players will stockpile these and use them for themselves or sell at a premium. (4.0 default change)
- If we remove the ability to craft the old gems, make them unrecoverable through mornstoning, and make the existing supply of them unable to socket, nobody will be able to obtain old gems except people who already had them in the gear and even then, you won't be able to upgrade that piece of gear if you want to retain those gems. This is really the only way to prevent new/average/undergeared players from getting price gouged, but this still isn't a perfect solution because it doesn't really allow them to catch up to the top 5% stat wise in the same manner.
Regardless of how you look at it, 4.0 is an awful patch for pretty much everyone but the top 100/established/wealthy from a balancing standpoint. All of the potential solutions make it harder for people towards the bottom of the spectrum.
Frigid tracks gem still exists for belt slot, it reduces frigid tracks cast time by a percentage, not a flat amount. There isn't a reason to keep both old and new frigid tracks gems.
zeal gems are not very balanced in a meta of inflated critical bonus and rate, zeal itself was progressively nerfed in essentially every patch from 4.5 onwards for this reason
More than likely they would create new recipes to make the old gems with the new mats, having two different material systems is bound to be extremely confusing and not to mention, annoying to work around.