People seem to be forgetting that mythic dungeons have a weekly lockout. You can't just farm them endlessly for 840+ hoping for warforged (which is capped at 850 until raids open). Mythic+ also doesn't open until raids do so you can't count those either.
If you want to have the best possible gear by the time the raid opens then you'll buy the ring.
PS the market value for enough obliterum to upgrade a crafted item to 850 ilvl will be more than 250k in the first few weeks.