Well, most popular stablecoins are centralized. How would "De-Fi" stay decentralized this way? It would be more like "CeDeFi" (Centralized + Decentralized Finance) to me. Regulators have already won, bringing the single points of failure from traditional banking right into "De-Fi". Truly-decentralized "De-Fi" projects (if any) are scarce these days.
I'd say the future of "De-Fi" is dark. But I know many will disagree with me. To each their own. At least, Bitcoin is getting "De-Fi" features (via sidechains and L2). Therefore, not all hope is lost...
I think with DeFi it's more of the platform allowing financial activity without centralized entity intervention, although stablecoin is centralized but the dex won't freeze a tx because it's supposedly to be permissionless protocol.
So I think they're still defi despite using stablecoin as a trading pair for perps platform or still provide stablecoin swapping for dex in general.
On the other hand, the DeFi platform can always provide route for decentralized coin swapping to another decentralized swapping without stablecoin at all.