I think "these days" is simply plural for 'today', which, technically, is impossible since there is only one 'today'.
It represents a string of current, or recent, days, without having to bend the brain to discriminate to the the exact days.
For instance, you've experienced a good event that you recall happened more frequently 8 years ago. More accurately, you most recently experienced it last Friday, Saturday, Monday, Wednesday, Thursday, and Friday.
So, instead of saying ""I'm glad that good stuff can still happen last Friday, Saturday, Monday, Wednesday, Thursday, and Friday"
you just say "I'm glad that good stuff can still happen these days."
Saves an awful lot of typing and bandwidth, IMHO.