If the Bank of Canada wants to save money on printing $5 bills, they should just make the upcoming $10 polymer note in greater quantities and have a perforation strip down the middle of the note that lets you separate it into two smaller notes worth $5 apiece. A really creative design would make this idea work.
They won't print $10 bills in greater quantities because its demand is in decline (it has been for years, maybe even decades, in fact) and is becoming scarcer. The most that $10 bills can buy nowadays are fast food meals (McDonald's, A&W, etc.) and items at Starbucks or other coffee shops. I'd say, in a few years' time, meals at McDonald's will end up costing more than $10 (including taxes), and the number of $10 bills in circulation will eventually tank. And it won't be long before our beautiful purple banknote ends up going the way of the half-dollar. Plus the majority of people, I think, that use $10 bills are teenagers or children who receive allowances and will spend them on fast food or coffee shop items. $10 bills are more of an allowance bill for now, it seems.
Seriously, what's the point of putting greater quantities of $10 bills in circulation when really they will become redundant in the future?
Of course, the $5 bill is losing demand as of lately, so maybe we are heading into an era where vending machine items may end up costing more than $2, and the day will come that a $5 coin may be introduced. Plus I have a feeling that the upcoming new $5 bill will probably continue with printer letter H, which I think is likely the highest printer letter for the CBN prefixes, and when the printer letter H nears the end of the run, we may then see a need for a $5 coin.