In an ideal world with a decent project and team setup, there should not be much difference. If, however, there are a lot holes to plug with every change (eg missing tests, process overhead, manual integration or deployment), productivity will suffer.
But you specifically mentioned focus on developer productivity — note that developers might be more productive if they work on stuff they want to work on too (eg. developers not familiar with "modern" JS frameworks might be motivated to work on a SPA using them). Perhaps you can establish better what your developers would be more productive at long term by doing a survey to establish what technologies are they interested in learning, and what they feel they know best today? And then you can make a call for what will be only a moderate push forward (i.e. do not have everyone do everything anew).
I get your point with hype but I think SPAs are long past the hype today — they have their use, and can provide some code structure clarity and organisation as well. But to get to a seamless experience like gmail.com is a gargantuan effort (the same holds true for non-SPA web pages, but the focus there is different, and they can be more efficient even if they are not seamless).