Dershowitz is not saying that nothing the President could do is impeachable...
I am not 100% sure at this point whether frustrated critics just don't understand the Dershowitz argument, are subconsciously or consciously refusing to accept the argument, or purposely straw-manning the argument to make it appear worse than what he is arguing. But whichever excuse they have, they are getting it wrong.
Now do I think Dershowitz goes a little too far on the absolute nature of his argument? Sure, but the underlying argument couldn't be any more sound, in terms of real world workings of our political system.
What Dershowitz is arguing (in a nutshell) is that the entire system of our government is a quid pro quo regarding garnering power and those who have it remaining in power. The basic reality is that our politicians are constantly running and campaigning for their next election (whether that be keeping their seat or moving up in the world). As much as they would love to tell you that they do everything for the good of the nation, nobody would (and nobody should believe them). The entire manner in which our Government is run (good, bad, or indifferent) is based on mutual relationships that exist within the scope of political quid pro quo.
Take the history of unions for example. Unions could collect dues, and then use a portion of those dues to back Democratic candidates for a variety of office. In exchange, Democrats in power would work to create laws that make it easier for unions to exist and for unions to collect dues and they work doubly hard to prevent laws that make it harder for people to unionize and to collect dues. The more successful Democrats were in controlling the laws, the more successful unions were collecting dues, and the more support (financial and otherwise) the unions could provide for these Democrats to be elected.
On the flip side, many corporations or corporate owners will back GOP candidates, hoping that they pass laws that might give them tax breaks, reduce red tape, and perhaps even limit their employees ability to go through a costly unionization process. If these laws pass, these GOP backers will make more money, and in theory be able to do more to support GOP candidates in their quest to win elections.
Now both sides will tell you that the real reason that they are doing this is because they believe in the causes (not because it helps them get reelected). But regardless of what you want to believe, most everything politicians do has one eye on whether or not it will help them get reelected. Even how a Congress person votes (or voted) in the impeachment fiasco is analyzed by whether or not it helps or hurts their election chances.
Imagine demanding that all of these basic fundamental gives and takes of our political machine are somehow akin to illegal election interference, or a corrupt quid pro quo that should land them all at the short end of an impeachment or removal process?
As obviously silly as that sounds, the question becomes where do we draw the line?
With half the country seemingly agreeing with the political considerations of every single politician, it's difficult to prove that any politicians does something entirely to get reelected. Even if they did, can that possibly be considered to be illegal? After all, part of their jobs is to keep their jobs. If they feel that they are the best person for the job (and most probably do) then they are allowed to openly campaign, openly seek out donors, openly vote for causes that will help them be reelected, and use their office to help that in anyway that isn't tangibly and defined as illegal.
In other words, Donald Trump asking about Hunter Biden in Ukraine is no different than Democrats supporting unions or Republicans supporting corporations. We all know that such support comes with a kickback of sorts, but as long as there is a plausible political argument to be made that they believe it's the right thing to do, then how can it be considered illegal?
The harm isn't in any argument that politicians are beyond reproach for any forms of campaign violations. There are literally
hundreds of laws on the books that specifically address what you can and cannot do. The harm is when you toss aside the pre-set laws and regulations, and allow partisan politicians to make an argument that their political opponents are acting in bad faith and therefore are breaking the law.
That, ladies and gentlemen, is the line that nobody should ever want to cross...