There are many problems with Proof of Stake (inflation, centralization, etc). Cardano mainnet doesn't even exist yet. Smoke and mirrors.
The Mainnet has 2,289 active stake pool operators, which comes out to >99% decentralized, with 71.09% of the total ADA in circulation being actively staked on those pools ($26.83b/$37.7b). It has an incredibly active and healthy mainnet...
Nothing useful is a HUGE stretch - unless only smart contracts are 'useful'. The ecosystem is new but growing rapidly. Let's circle back to this comment in 1-2 months?
Cardano main net has been around a long time. It just doesn't support smart contracts yet.