I suspect Canva is willing to offer Affinity for free because it holds Adobe's feet to the fire and forces them to compete on Canva's home turf: the nonprofessional design market.
There's clearly a funnel for Canva Pro upgrades, but (to my knowledge) they've never paywalled formerly-free features, and it seems to be a profitable strategy so far.
OTOH some sources report Canva paid like $400M for Affinity. Even for a company like Canva it does seem significant. They might need to adapt their strategy if the investment doesn't pan out as expected.
If they can't monetize the product with ai subscriptions they sure as hell will end up monetizing their users and their content.