Update React for SPFx1.4 for SP2019.
If we don't upgrade React to 16.8 atleast then we can't use react hooks,
> so can't use latest fluent UI controls as they all use react hooks
> can't use latest validation libararies like react-hook-forms as also rely on same
> can't use latest version of PNP.JS/controls
That makes mostly like an abandoned platform rather. Most of the customers with us are on-prem, so this is essential for the future SharePoint IMHO