Any games that you have bought and paid for, you can continue to play.
Any games that you have claimed as part of being a Pro Member, you will lose access to if you choose to end your Pro Membership.
But I understand the confusion with no official release date for base. My best guess would be that you will still have access to Stadia if you end your Pro Membership. If Google denied Founders access at the end of the 3 Month Membership I think that would a huge black mark on Stadia, one that they probably would not recover from.
@SanSatan : As @SeaTroot has explained, if you choose to let your Pro subscription expire, your account in effect switches to that of a base account, even though that bundle is not available yet. That is in effect what happens to your account. Any purchased games continue to be playable albeit at 1080p in stereo, and you will no longer be able to play any titles in 4K or with 5.1 audio. Also any claimed titles will no longer be playable unless the subscription is renewed at a later date. If you do choose to renew your subscription then saves and achievement progress for those previously claimed titles will still be active and you will be able to continue from where you left off.
This is in the same way that would have happened already for anyone that purchased either a Founders Edition or Premium Edition bundle, then purchased some games and then decided to return their bundle within the timeframe of the return policy. Their account would still be able to access their purchased games but they would no longer be able to play any previously claimed titles or take advantage of any Pro discounts.
I also believe that a Pro subscription permits a larger number of screenshots and clips to be stored (I forget the numbers involved), but you would not be able to store more than whatever the limit is, whatever that is, 100, 500 or something I can't recall off the top of my head.