I'm not sure I agree that SPA's are a mistake - specifically, I believe loading new pages isn't a nice experience regardless of how fast they are, and that sharing client-side data across multiple views is more of a pain if it's not all on the same page.

However, as with other newer technologies, we shouldn't treat it as an undisputed best practice to adopt SPA's. They have pros and cons, and it's worth thinking through how SPA's and MPA's would affect your product before committing to either approach. In particular, an MPA page's Javascript bundle could include only what that page requires, as opposed to the code for every page on the site, which could make a huge difference to performance.