I do think the part about arresting a sitting president is legal sophistry.
It may well be true that a sitting president cannot be arrested, but that hardly means that abuses of power are irrelevant, and therefore - it is implied - need not, or must not be looked into.
Surely there is a process for impeachment. And assuming that the abuses in question are both true and scandalous enough to warrant impeachment, or a public outcry large enough to force political change, there is a strong argument to be made for the imperative, moral if not legal, to bring these to light.
To keep the conversation limited to remedies and/or penalties, or lack thereof, is to give no regard to the societal and moral underpinnings of the legal system. Legal loopholes are more forgivable or even appropriate when defending a common crook, less so when discussing those who hold the highest office and power in the land, and the abuse thereof.