For EC it will basically never be a problem because the moment that they "move the curtain" (i.e. change business class seats to economy comfort) and that Golds (not Silvers) can get them for free lines up almost perfectly. There is a window of only a few hours where there are more EC seats available but Golds still have to pay for them. I don't know exactly what the moment for moving the curtain is but it's on the order of T-80 while Golds get such seats for free at T-72. The only way it's a problem is if the business cabin is so full that they only have the default row(s) of EC or, in theory, have to move the curtain backwards which I don't know if that ever happens.

For the purple seats I have literally never seen those substantially filled before OLCI, again ignoring the inaugural A21N flight. In fact I have another A21N flight booked and there are only 3 such seats unavailable, all next to eachother (7ABC).