It looks like I'll have to get Shadow cards now

You don't have to. I have just researched the topic. They directly say: it is better to have a strong meteorite card (that will help you to reach a higher rank) than a weak shiny card. So nothing has changed:

1. upgrade plain cards to meteorite ones
2. try to reach higher ranks

What's changed: nowadays, playing decks full of shiny cards in lower ranks is less profitable than playing decks made of meteorite cards in higher ranks. UPD: and, of course, consecutive wins bring good bonus after the update. Of course, if you play shiny cards in Mythic, it's even better.

So don't spend GODS for fusing shadows. Now it's perfect time to earn GODS on selling Core cards. Then, buy meteorite cards you actually need for your deck.

Imo