> It will cost you 10x what you’re paying now, it will be much more brittle and harder to fix.

I say this as someone who's done a lot of freelance work: No, you can't bullshit your clients like this. You may be able to tell them that if it's not an actual lie, but in your case, if you actually would endure such costs and fragility, it speaks volumes about your agency's skill levels.

If you're in the business of selling websites, maybe it's time to actually learn how to write a good SPA rather than bury your head in the sand, because yeah, clients do actually want those.

Maybe I should’ve made it clear that the 10x is an example. In most cases it would be 2x to 4x, the cost for implementing two apps instead of one.

Of course the cheaper option (monolith app + turbolinks and js sprinkles) could be framed as a discount.

And I can imagine how many agencies would not mention the expense of maintenance, because they’d actually want to lock in the client first.

10x the cost!? Well, I have a quote from your competitor that's a fraction of that amount. If you keep quoting unreasonable prices, perhaps I should rethink our business relationship and move elsewhere.

Sure, you can say that. You will lose your clients to the agency down the road who happily use React and say "no problem." Especially once it turns out that it _doesn't_ require 10x the time and budget and your prospective client tells everyone you bullshitted them.

...and it breaks linking to URLs and backspace.

Turbolinks? Huh? You have to write your code keeping turbolinks in mind because certain onload events will only fire once but you use turbolinks specifically because you want those things to work but want the speed advantage of not doing a full page refresh.