The totally reasonable practice of "I lost the game, so I'm going to flip over the table and pull a gun."

Appointing people based on party loyalty is always cited as one of the major reason the Soviet Union became a slow-motion train wreck. It's not something America should emulate.

Not to mention that packing the courts could well be interpreted as an open attack against the separation of powers

superficially this argument seems reasonable.. but my limited understanding of the history of the Supreme Court of the United States says that there have been substantially different eras, and substantially different rules in those eras, for this same Federal body. Needless to say, in a "two party" political system, the details of what each of those two parties represents has also changed dramatically.. i.e. what is called conservative has changed quite a lot, many times.. same with "liberal"