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.
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.