You lay out an impossible challenge for Canva, there is no way they can prove that they will never add a subscription service or different charges in the future.
What exactly do you expect from them? Would you prefer they just kept charging you for the product? That still isn't a guarantee that they wouldn't move towards more paid features and subscriptions in the future.
> Would you prefer they just kept charging you for the product?
Yes, exactly. Knowing that my interests, my consumer spending choices, are the direct feedback path to their profitability is one of the only ways to provide some concrete assurances that they'll be building for the customer's needs and not for data collection, AI shovelware, or some other play.
People complain about Adobe's subscription model but it's superior to free-to-play consumer software because it still keeps an alignment between the consumer interest and the company's income. Despite its other faults, you could even argue that a consumer subscription model can be better aligned than single purchase software because the customer needs to continually choose to pay the company for its use and it incentivizes continually improvement and competition.
What exactly do you expect from them? Would you prefer they just kept charging you for the product? That still isn't a guarantee that they wouldn't move towards more paid features and subscriptions in the future.