Exactly this. Supreme Court expansion is a zero sum game. Once it starts, it will never end - the party in charge will be under continual pressure to expand to re-gain the upper hand.
One thing the proponents of expansion don't consider is that the ramifications extend well beyond just the immediate effects of expansion. Specifically, the stability of our legal system depends on adherence to stable legal precedent. And, the stability of our government and our economy depends upon the stability and predictability of our courts.
The whole purpose of expansion is to immediately be able to quickly turn the courts in a totally different direction to "get what they want, now". Thus, precedent literally won't mean anything anymore because the prior loosing side in every case, major or minor, will want to re-litigate the issues of their cases in hopes of a better outcome every time there is a change in the composition of the Supreme Court.
It is all a moot case, anyway. The Democrats don't have the votes to expand the court. Furthermore, I'm not convinced that Congress can even term limit Supreme Court justices without a constitutional amendment. If they try to legislate it, it will be litigated and we'd eventually see the peculiar spectacle of the Supreme Court deciding if it is legal to limit their own positions. The Constitution doesn't provide for it in Article III, so the current court would say no and look self-serving while doing it.
Like this works so well now for Congress.