It strikes me that "Eternal September" is an older generations version of "Enshittification" — an expression of the same anxiety about change, the same entitlement to have everything stay the same.
How is enshittification not objectively bad?
It's the logical progression of platforms, where they start abusing users openly.
"Abusing users" in eshittification is fundamentally about violating expectations/norms, which is what Eternal September was about. Before "Eternal September", Usenet users came aboard in small waves and acclimatized to the local culture. This was the norm on Usenet. Once Usenet opened up, users joined so quickly that they could participate in the network with no shame without acclimatizing to the local culture. Likewise, when more ads appear or previously-free API surfaces begin costing users, older norms/expectations have been violated.
It's also not wrong to feel angry when expectations change. If a Dollar General in your area raises prices, it puts pressure on local residents who may have just barely been staying solvent. Through no fault of their own, those residents now have even more financial stress on them. This is why economics is a complicated field; things like interest rate regimes have broad knock-on effects on the economy and create winners and losers that under different regimes may have never emerged.
Usually a restaurant neither has a captive audience nor a growth imperative (unless it is a PE-owned chain restaurant).
It's not like by adding a moniker on this phenomenon ("enshittification") or by upvoting or downvoting arguments we agree with that we can change the phenomenon anyway; pricing and economics are a distributed exercise.
And with you there, it's already starting to sound like a buzzword to me. The first step on the road to losing all meaning.
"Eternal September" was not about monetization, only about the declining level of technical clue among (college-student) internet users; it wasn't about (commercial email) spam either.