You can still buy bulk Legos to mix and match from Lego. And similarly pricey and collector oriented Lego sets existed 20 and 30 years ago as well.
Legos releasing single build sets that are clearly targeted for adults (look at the 18+ age statement) does nothing to harm you - it's targeted a different consumer demographic.
It's like Taco Bells now serving alcohol or Costcos now selling Asian groceries. Companies will not stay stagnant and will look at additional opportunities to expand to new buyer demographics.
Exactly, and like, if one literally looks up "Lego catalogue" and actually read it you'll get a bunch of sets - even the brand tie-ins, which aren't new at all - and these are basically the same types of sets I grew up with and would happily build and then take apart as a kid to do other stuff with...
Lego also has "3-in-1" sets that come with dedicated instructions to build different possible configurations out of the exact same pieces, which seem like a cool way to encourage kids (or anyone :P) to then veer off into their own building experiments.
There were also two 20-in-1 sets recently, both very reasonably priced. Those were huge hits in my house.
Same here much to my parents chagrin - they thought buying me Lego sets would teach me to follow written instructions but joke's on them.