Bronze & New players!
How will you overcome these changes?! How can you earn more DEC if you don't get DEC? D:
Here's the bad news, you won't without investing something. Either time, or money. You can still earn even with these changes!
You can still earn even with these changes!
All is not lost yet. Even if Card % was 0 at the start, credits can get you there. You'll have to play the market. If you only want to invest the initial cost of the Spellbook there is still a slow but steady path forward. With credits you can buy cards. At first it doesn't matter what you buy with them. If you want to resell them you can, or you can burn them to get the DEC you need. Why do you need DEC if you can use credits? Hang on I'm getting to that!
Yes YOU!!!
Here's a somewhat daunting suggestion for new players. There is a more or less underutilized tool in @Splinterlands called Tournaments. THAT'S WHAT I SAID! Tournaments! Take a look at this:
I know it sounds crazy, but you can legitimately earn DEC from low cost tournaments. Just check the rules. Invest the DEC you make from selling or burning your cards in Renting AND Tournaments. It's a more viable option than you think.
I know from experience. You see, when I was very new, I decided to change things up a bit, and entered a tournament. A lot of them have prizes even if you don't come in first. I came in 35th place and won my entry fee back and then some.
I know what you're saying, well what are the chances there's going to be tournaments like that regularly? Well actually several people and guilds do post tournaments regularly. 
This isn't a push for his tournaments but this is an example. How about a free weekly shot at 500 DEC? Does it solve all your problems? No. Is it easy and quick? Definitely not. But it is hope. And a possible path if you're hell bent on not spending more than the $10 bucks for the Spellbook.