ExtJS / Sencha

ExtJS was the enterprise JavaScript framework — data grids, form builders, layout managers, and a widget toolkit that predated modern component libraries. Sencha bought it, added licensing costs, and the open-source world moved on.

React with Material UI provides everything ExtJS offered with a larger ecosystem, better documentation, and no commercial licence. The data grid that was ExtJS's killer feature is now available from MUI, AG Grid, and TanStack Table. The migration cost is real, but the licensing cost and developer scarcity of staying on ExtJS is worse.