Thanks for everyones' feedback on this change - we've updated how we're approaching this and increasing the limits, please read the below & linked community post
Shopify devs, we heard you on the recent change to metafield limits.
After discussions with the community, we've revised our approach: 128KB for JSON (not 16KB), 64KB for all other types (existing limit).
Additionally, existing apps who use JSON fields will be grandfathered at the current 2MB limit, and we'll have an intake process where new apps can apply for higher limits where necessary.
Thanks for the detailed feedback. It made us revisit our assumptions and improved our POV.