Steam Cards from Summer Sale

It says 'Valid until 8/5/2017, 7:00:00 PM' meaning after that date they expire.

Yes they will disappear and you will not be able to use/sell them anymore.

They will vanish from every phase of existence, into the cumulus and beyond.
You won't be able to craft from, sell, or buy them.

yes they disappear...I got rid of my cards yesterday
I wouldn't wait till the last second xD

sad had over 100 cards in inventory but was trade-banned because of switching my steamguard to another phone.
thought i would be able to convert them into gems, even if i couldn´t trade them or craft them (missing some for complete badges).
now valve stole the gems from me

Not sure what tough process made you think that Valid means that they will magically become more expensive after said date :p

They been dissapearing since 2013 dude