Product owners & users increasingly expect a UX that demands JavaScript. Typeahead autocomplete on tags, lists that filter, lists that drag, lists that reorder by dragging, in place editing, baby-butt smooth transitions, timers that count down, counters that count up, alerts that dismiss, modals, accordions, tabs, navigations that hamburger, signup forms that validate email uniqueness as the user types, maps with overlays, charts, printable charts, printable charts that are also interactive, calendar date pickers, a video or sound player that continues during site navigation, direct messages that creepily the user know when the other person is typing, and that had all better work in the desktop and on mobile phones at 60 frames per second.
Users expect this and they don't care that they're using a server rendered page, nor do they care if the routing takes place on the server or the client.
Luckily, dynamic user interfaces do not require single page apps backed by JSON API's.