And ...
“Here are a couple of technologies which would allow us to speed up the experience at a very low cost. I recommend we try these first.”
Turbolinks for the win :)
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.
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.