That's really just the "end of the Supreme Court" argument. If you pack the Supreme Court once, it'll happen every time a new party has the senate and the white house.
Especially since you yourself suggest packing it to give an immediate "Democrat" majority, when it's incredibly likely, even without a new nomination, the Supreme Court remains republican for at least 8 years.
Personally I think this is what democrats (and people like you) are hoping for. It'd give an excuse to pack the court day one and maybe not immediately lose all public support.
That depends on how the court is packed, and how it functions after. It's possible that court packing could be seen as a success, even if each side alternates at packing, just because having many justices can minimize some of the court's current limitations.
To put it another way, the size of the House, Senate, and Presidency are determined by the constitution. The Judiciary is the only branch where it's size can be increased without a constitutional amendment, which could favor populism, regardless of who is in power, if it's expanded significantly.
Being as small as it is could speak to how the government has a tendency to concentrate/centralize power, even if it's relatively easy to spread that power out.
You are incorrect on one thing. The size of the House is not determined by constitution. The only things it guarantees are that there’s a census and that each state gets at least one.
Especially since you yourself suggest packing it to give an immediate "Democrat" majority, when it's incredibly likely, even without a new nomination, the Supreme Court remains republican for at least 8 years.
Personally I think this is what democrats (and people like you) are hoping for. It'd give an excuse to pack the court day one and maybe not immediately lose all public support.