The reason (IIRC) for Greybull Precision not offering a 6.5-20, besided weight & size, is elevation travel. The 4.5-14 will travel farther than a 6.5-20... Just food for thought.
Yes sir, I also believe Leupold will be around for the long haul as they are finally starting to pull their head out of their rear...

with the MK6 & MK8 lines..$$$$
The Holland ART is an intriguing idea & would be worth looking at as well.
The Greybul team, especially John B, has been there & done that as much (or probably more) than most anyone I can think of. If they use them, it's for a reason besides marketing
I have thought about the Vortex line many, many times... Darn neared picked up a PST last week when I found one on the shelf... I'm still struggling with doing so....