Nope. You have no idea if plankton will slowly die out because of pollution or acidification, or for some other reason.

Maybe plankton stocks will increase. But you don't know that. You can't see the future anymore than anyone else can.

If extra CO2 is always absorbed by plankton, then the increase in CO2 we've recently seen would be impossible : - D

Existing evidence kinda shows you're wrong :-)