18 bucks per month for Acrobat DC is a hefty price tag for those like me who are occasionally creating fillable PDF forms. Even worse, I do it in my spare time for a non-profit organization. (Also, after having been annoyed for years by Flash and AIR, I find the idea of installing whatever app from Adobe really hard to swallow.)
If you need the full range of PDF forms features including validation, submission etc, okay, go for Acrobat DC, no need to implement all this in Affinity. But IMO, most people just want to add the ability to fill out PDFs on the computer before emailing or printing them. This limited feature set should be much easier to implement, maybe even in as little as a couple of weeks.