I think it's simpler than that. Canva's primary competitor is Adobe, and Adobe's remaining advantage is with creative professionals. That's Adobe's core market and their core revenue stream.

It's a classic "commoditize your complements" play. Canva remains profitable without charging for Affinity, but Adobe can't stay profitable if they stop charging for Photoshop/Illustrator.

The business justification works without imputing any more sinister motives than that.