Will Eth 2.0 be more decentralized than Cardano?
Will Eth 2.0 be a hard fork where there will be Eth 1.0 and Eth 2.0? If not then will Eth 2.0 still need to rely on Polygon Matic for cheap fees or Polygon becomes obsolete when Eth 2.0 is released?
Can't tell for sure whenever ETH 2.0 will be more decentralized than Cardano or all the other way around. I'd say they're neck-and-neck when it comes to achieving security and reliability. Cardano is backed by academic research, so there's that. According to the developer team, the Ouroboros PoS consensus aims to achieve scalability and cost-efficiency without sacrificing decentralization. We're going to have to wait until ETH 2.0 is launched in order to test both blockchain networks' level of decentralization.
As far as hard forks are concerned, ETH 2.0 will consist of a multi-tiered launch process. Ethereum will become a hybrid coin first (PoW and PoS) until the network fully transitions into PoS (which is ETH 2.0). Polygon won't become obsolete simply because it's a sidechain with its own network architecture. After all, Polygon relies on the main ETH blockchain for security. The launch of ETH 2.0 is inevitable, so the best thing other projects can do is to adopt the latest trends in the crypto/Blockchain space in order to avoid being left behind in the dust.