Matchmaking was only open so many hours in a day. I don't know if the available uptime was region based, but at least in North America, it was open in the late afternoon/early evenings Monday through Friday. It was open for most of the day on Saturday and Sunday.
It's VERY MUCH a work in progress. Despite how many people are already playing it, they're actively changing it all the time, and patches can and will drop without any warning.
Nothing is stopping existing owners from downloading the offline installer backups, but existing owners will still have access to their games through GOG regardless. Delisting just means that the games will no longer be purchasable, so if there's a game you want to "own", as long as you buy it before it's delisted, it will remain accessible to you for the foreseeable future.