Feature summary:
Option for the API that, if set, causes the watchlist setting to remain unchanged if the user is already watching the page for a longer period of time than the period specified. This could be implemented as a new value for "watchlist" besides the existing "nochange", "preferences", "unwatch" and "watch". I'd suggest "nodecrease" or "onlyincrease".

For example when setting "watchlist:onlyincrease" with "watchlistexpiry:30 days":

• User is not yet watching the page: set watchlist expiry to 30 days
• User has 5 days left on their watchlist for the page: set expiry to 30 days
• User has 42 days left on their watchlist for the page: do nothing
• User is watching the page indefinitely: do nothing

Use case(s):
Useful for any script or extension developer who uses the API to let the user perform watchable actions and the users of those scripts and extensions.

Benefits:
Prevent unintentionally reducing watch time.