While i've been in the situation where i needed a specific gear, had enough crystals, but the darn thing wouldn't show up in the shipments. As much as that sucks, i still wouldn't recommend stocking up on gear you potentially need in the future. Imo it's better to use crystals for direct inprovements to your roster instead of investing in gear that will remain unequiped for an unknown amount of time.
Also, like nastynas said, you don't need the mk6 furnace nearly as often as the mk5. https://swgoh.gg/db/gear/143/mk-6-cec-fusion-furnace/