I personally only upgrade to gold league on the day before or day of the season ending. Without the proper cards, I think you can still win but each jump in league becomes much harder.
If I upgrade my rank 1 day before, I can collect the bonus chests from quests rewards twice. So long as I burn my energy capture rate, I won't have to worry about fighting.
1 more quest reward chest and maybe a few more DEC per win (but more loses so might even out)
Harder Fights and tournaments are more expensive for certain ones