$1M ARR with an 8 person team is by no means "doing great". Assuming 20% of income goes to overheads (such as hosting, marketing, legal, accounting), that leaves a salary of $100k for each staff member. Even if they had 0 overhead, this is still only $125k each -- that's hardly big money.
Also, in reading the post - I find little to object to. They're not changing their license. They're not doing anything to prevent self-hosting.
They're making it clear they're not offering support to people who don't pay (completely reasonable), and gatekeeping a few features that add value - while still giving away a huge amount for free. That's a fair deal to ensure the company remains viable, which is in everyone's best interest. In practice, this decision is unlikely to materially impact anyone.
Open source is a gift, not a right, and certainly not an obligation - and from what I can see, they're not doing anything here that undermines the Open Source-ness of their offering.
FWIW, I'm not sure you'd find an alternative analytics offering that's both commercially viable, and "more open" than what Plausible are offering today.