The way I see it, older Paradox games are better sandboxes than games, whilst newer paradox games are built to be games first and sandboxes second.
I don't understand why you divorce the sandbox genre into not being "game(s)". They are, and it's really weird to motion them as being mutually exclusive. If modern Paradox games aren't sandbox games, what exactly are they?
On your example of CK2 vs CK3:
CK3 in the state it is now, is nowhere near as being good as CK2. And to be fair, you can chalk it up to CK2 being heavily pegged upward by the DLC it has, and CK3 barely having any... but in the same amount of years of CK3 being released have passed, CK2 had more DLCs out by now. And almost all the DLCs for CK2 within that aforementioned timespan, are leagues better than the DLC currently released for CK3.
I'll give CK3 a little bone; baseline CK3 definitely incorporated of minutiae things (and emphasis on minutiae here) from CK2+DLC to stuff it up on release.. but CK3 misses core gameplay mechanics that make it subpar compared to CK2.
- No playable Merchant Republics
- No dynamic Nomad system for Central Asia
- No unique government structure and mechanics for the Eastern Roman Empire
- No College of Cardinal system
- No joinable Warrior Lodges for pagan characters
- No Canonization of deceased characters of high virtue by the Papacy
- No coronations
- Complete lack of Secret and Public Societies characters can join
- No trade routes
- The basic inability to serve as a commander in your Liege's armies
- All the army unit models being the same Infantry model, instead of changing dynamically based on the amount of units you have in it (so for instance you had more heavy cavalry than all other types in your current army; it'd show a nicely made and detailed graphic unit of a heavy cavalry sprite on the map. Try the same with CK3 and you'll get the same basic footman unit sprite everytime)
..and many, many more things. So your claim that Paradox is making games with better "gameplay" in mind.. is truly bizarre.
To be honest, all the above of what I said should've probably been put in the CK3 or CK2 thread, but really it just serves as a case-example of Paradox as a whole.
Paradox does not care about gameplay. They care more about making gradually oversimplified games, in which they get picked up by YouTubers to farm easy content off of and make bank from the people who'll buy their games for the meme value, then stop playing it a few days later after being bored. Victoria 3 proves this readily as more and more people are beginning to drop it, and the Steam reception reviews for it only going up by 1% from 63% to 64% in the weeks it has been out already.
That's the real transition of Paradox game design, not them making better "gameplay" in their games.