you don't have to make a monolith web app, or use React, Angular et.al, or transpilers, not even JQuery. State can be managed by observable pattern or event listeners. example:

  app.on("new_private_msg", updateMsgBadge);
  app.on("new_chat_msg", updateMsgBadge);

  function updateMsgBadge() {
    this.badge.newMessages = 
      PRIVATE_MSG.reduce(msg => msg.unread)
      + CHAT_MSG.reduce(msg => msg.unread);      
  }

Parts of the app can be abstracted into micro-services. For example the signup can be it's own micro-service. Or using the plugin pattern each widget can be independent/untangled.