I wish SaaS companies (not just DatoCMS) would learn that developers will not adopt their products if they don't have source access.
This product could have easily been FOSS with it's pricing model based exclusively on support/hosting.
If you want early-adopters - be hacker-friendly, not money/data-hungry.
I do somewhat agree with the opinion of making it open source. Imagine how much better the platform could be with constant developer input. But I understand that might not be an easy decision to make.
I've used DatoCMS and I actually really liked it. However the pricing model is quite steep for the service it provides. I feel that if open sourcing isn't in the horizon, at least consider expanding what the Free and Basic models provide.
The thing you offer is a fancy editor for statically generated websites, but for the prices you're asking, I would expect you to host the site as well. Also, looking at the videos/gifs on your website, it seems to me that the editor isn't even truly WYSIWYG. Sure, the text-areas are WYSIWYG, but you're not editing the content in the templates themselves, which would be way more useful.
I also want to second the suggestion to make this FOSS. I know you fear for the economic repercussions, but unless your product is unique and super game-changing (which IMHO it isn't), people will mostly pay for the service they get on top of the product. This can still be a differentiator, even when the software itself is also available as FOSS.