Well, this is how you become what you're fighting against.
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.
https://time.com/5338689/supreme-court-packing/
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.
So, yes, the total number isn't fixed but the apportionment is.
If McConnell violates this precedent that he set, I would strongly advocate for Democrats (whenever they have the Senate) to remove the legislative filibuster and increase the size of the SCOTUS to 17.