sad mtg cards A Whole New World: After Rotation – Innistrad Block Blue Magic cards depicting loss/grief :

To simplify Sinkhole , just perform any advancement on the board or obtain any extra bit of card advantage then follow up with Sinkhole to disrupt your opponent

The cool thing about Polymorph is that you can also target your own creature to possibly hit the table with a huge Kraken/Leviathan such as Hullbreaker Horror or Inkwell Leviathan

This can lead to plenty of extra mill over the course of a long game, especially in Commander, where this adds each-opponent mill to any instances of single-target mill

It is also probably worth it in a well constructed Tempered Steel deck because it scales so well and already has sufficient white mana