Those 3 things are being addressed, I think. Decentralization is due to many different miners and full nodes, many different devs contributing code. Scaling is being worked on by centralized entities such as off-chain solutions either with third party custodians, exchanges, web wallets, points thingies, token thingies. Privacy, at least on the bitcoin level, is being done by such things as CoinJoin, JoinMarket, Wasabi and all the other privacy tech by other altcoins that supposedly focus on them like Monero and ZCash.
As you implied, these things are being worked on separately (and maybe together) by different groups of people. They'll eventually come together, but understandably some things are prioritized depending on the group or devs working on them. If you're smart, you can take advantage of all three right now.
Mining is highly centralized because of pools, centralized services are not part of the game and shouldn't be considered a scaling solution and privacy gains from centralized services is not true privacy.
You are right though, we can take advantage of and learn lessons from available centralized solutions. It is possible to re-define this project as an attempt to decentralize existent solutions to current shortcomings in bitcoin mining, scaling and privacy.
For Instance, you can imagine a built-in, on-chain coin join service for privacy. Actually, I've been contributing to such an idea a while ago and integrated it into my work lately.
