Perhaps in the future there will be a time where an 'old-fashioned' server-side app is a bad idea, but right now I think it's still a perfectly valid approach,

Perhaps in the future there will be a time where an 'old-fashioned' server-side app is a bad idea, but right now I think it's still a perfectly valid approach, and even preferable in some cases. Consider whether your app will actually benefit from being an SPA. Does it negatively impact the user experience to have page refreshes? Or does it provide significant benefits to do things client side? If the answer is 'no', you might as well take what is currently still easier and more standardized, and go for server-side. But the next step is to ask these questions about parts of your app. In those cases using React or Vue or something similar for just those parts might be a good idea. And if you've already separated front- and back-end, it shouldn't be too difficult to realize (and it's a good reason to use React server-side even if that's all you use it for initially!).