There are limitations. The information requested must be pertinent to the investigation, which is a matter of judgement. And the specifics of the case matter, which is why the SCOTUS would likely get pulled in
In this case, the question is whether or not the President can blatantly ignore a subpoena while under criminal investigation. I'm pretty sure "come talk to us about it" is a legitimately pertinent request. Trump is arguing that he has no legal obligation to even respond to a subpoena, because he is above the law. Not that the questions he will be asked aren't pertinent, not that the investigation is prejudicial, but that the law simply doesn't apply to him whenever he decides so. That's not separation of powers, that's corruption.
And are we really surprised? Trump's entire presidential campaign was built on the idea of ignoring the ordinary and customary rules of an election, and that's part of the reason his supporters loved him. A president isn't supposed to bang porn stars, and yet here are. A president isn't supposed to support racism, and yet here we are. A president isn't supposed to take bribes, and yet here we are. A president isn't supposed to work with hostile foreign powers to sway elections... shall I go on?
And some people absolutely love these things about him, because they think it makes him "different" from all of those "elitist snobs" that used to run the government. You know, those elitist snobs that actually felt compelled to follow the law? The ones who will never "win" because they choose to follow rules that it turns out you can just ignore without consequences?
As long as one particular party in government actively chooses to endorse criminal activity, American democracy is dead and gone. I won't be surprised if the midterms are a republican landslide in every state, with coordinated Russian hacking of election machines and GOP governors conveniently continuing to destroy all paper receipts of voting records. I won't be surprised if a republican congress refuses to do anything about Trump running for a blatantly illegal third or fourth consecutive term as president. I won't be surprised if we end up like Russia or Cuba, where the dictator apparently gets 90% or more of the popular vote despite nobody on the street admitting to voting for him and widespread protests about corruption.
Like what happens if Trump fires four supreme court justices and appoints his four children to fill the seats? Would congressional republicans do anything, or would they just continue to say "we don't agree with the President's methods but it's not our place to intervene" like they've been doing for two years now? What if decides to just cancel the EPA and allocates all of its budget to Exxon/Mobil, who would stop him? Fires Mueller and his next three replacements until the investigation disappears? Seriously, is there anything he couldn't get away with at this point?
Kavanaugh is just another stepping stone in this process, another way to ensure ultimate power forever by appointing someone who will never interfere with your dictatorial rise. Arguing about the details of Kavanaugh's voting record is a red herring. All that will matter is that Kavanaugh will support Trump's immunity from any criminal prosecution, protecting him from the system of checks and balances that the Constitution requested, but that we no longer believe in.