Well from my perspective it should be possible to buy keys as long as there is time to redeem the chest, that's the whole purpose of having items in cash shop, if you didn't managed to get remnants / was too late to gather needed amount you can buy it for cash. Now as a returning player that just jumped at the end of the event I'm left behind while the same upgrades would cost thousands of gold and take huge amount of time (and I still wouldn't be able to catch up) while I literally see the chest in my inventory.
Yes, maybe too late is too late, but that should be 11/9 when chests will be removed, not 3 weeks prior to that. And we are not asking to bring back remnants, we are asking for cash shop item.