Throwaway for anonymity.
I work at a place that is mostly like this, I can tell you that using html/css/javascript raw, with a bit of bootstrap is a nightmare with a SPA.
Menus are constantly broken, back button is a game of roulette, caching is constantly a problem showing stale data, xss and other vulnerabilities are ubiquitous.
There are modern affordances in many of these frameworks others take for granted.