Throw in a change of leadership or business focus and it's an easy short term boost to drop the many smaller customers and focus on the big fish who make the real money.
It's a common pattern, echoed over many industries, and while you might not see it being likely here right now, if the concept literally doesn't make sense to you, you need to look up some basic business ideas because it's a pretty valid concern.
This small $5 plan is obviously not going to make Planetscale very much revenue.