I just want to add my voice to the chorus here that I find the loss of Core to be a real shame.
Having just finished my first project on Dato, I have been really happy with the quality of the CMS and was literally a day away from purchasing a Core subscription for the project to move to production. However, the Professional pricing model is going to be a nonstarter for most of my clients, echoing what @dev1 said in regards to other services that I already have to charge for.
The Free Tier is actually really fantastic with Dato, but without the ability to add a collaborator, it’s basically impossible to give my clients access to the CMS (I use my gmail account to log in).
If there is no small-site pricing plan like Core available, I unfortunately will not be able to move to Professional but instead will have to move to a lesser CMS like Prismic.