As a technical person, a recent Y!G migrant and one who has set up Open Source Email List software, I believe I have sufficient experience to comment on the Price Change.
The reality is it takes money to run servers and to pay excellent people. The fact that the system has grown so much in so little time, and the system did not crumble means that Mark and his team has architected an excellent solution. An At Scale solution. Finding people who have this type of experience is difficult. When you find them, they're not cheap. I know, I'm one of those types of persons. ;)
Given my short time period on groups.io, the spirit of what Mark is doing is clear to me: he's not trying to rip people off. As I see it, he's continuing to build a scalable system, provide top-notch service and on top of that, maintain a family life.
From my experience (over 30+ years), you try your best at architecting and anticipating at-scale. However, for every order of magnitude in growth (10x), there are things that you could not foresee. My bet is that there's a lot of scrambling behind the scenes to ensure from the user experience, all is well.
I believe that Mark will come up with something equitable for the Basic plans. It may be that you have to pay by volume of email sent (e.g. bytes) averaged over a period of time but I don't know. ;)
- Mailman3 - see https://www.gnu.org/software/mailman/download.html