As an occasional and very long time user of Serif products, I'm very sad to hear this news. Whilst it may not be the same as Adobe buying Macromedia (remember them?) or Corel (was it?) buying Ulead's PhotoImpact (RIP), I still think that Affinity will soon move to a subscription only model (within the next couple of years). Once Serif has been sold to Canva, there will literally be nothing Serif staff can do about it.
I don't buy the logic that the Canva sale is required to invest in Serif to make the products better. If you look at Serif's accounts (available online as they are a company registered in England), they made around £16m profit out of around £32m in sales in 2022. Admin expenses (which includes staff wages) were less than £13m, so Serif weren't short of cash to invest in other projects or more staff (though, as a software dev/architect, more staff does not always equate to greater productivity, indeed sometimes the opposite). But the money was there already, assuming 2023 wasn't a complete disaster for them.
Basically, the shareholders have sold Serif for presumably a large payday. There was enough profit to invest in other stuff, should they have wished to. Now they've lost control of the company (or are about to), so are no longer in charge of their own destiny. What a shame for British software.