But even if it grows linearly in transactions, that depends on transaction growth which might not be linear... and that's still independent of what's tolerable for a typical user.

I was going to post something related, which was when I tried bitcoin for purchases, the whole thing involved way too much faff and was too slow for everyday use. Maybe for some purchases but not for everything I might do online. I guess if you had an account in bitcoin but then you're back to banks etc which might defeat the purpose slightly.