The threat of impeachment may be less of a deterrent than the Constitution's framers imagined.
They probably never imagined such a garbage pile of humanity as the GOP being devoid of any and all morals...
Well, no duh! Turns out, Founding Fathers weren't very smart or attentive. If they were intent on not creating a King, then they wouldn't grant a single, one person the powers of a King, like for example power of pardon (if they wanted the possibility of pardons, vest it in the Supreme Court, or the Congress, or its subset - a large enough number of people - or have a staggered commission be elected and unanimous / super majority in verdict reverse, to prevent or at least make a chance of corruption that much smaller.) They could have made it impossible for Congress to abrogate its powers to President or executive branch.
It also inevitably leads to "monied interests" or foreign powers, or all "enemies, foreign and domestic" to try and affect election of a single individual, so the visible culmination of that effort should not be a surprise, it could long be predicted and in fact in some books and TV series have been - we just mostly ignored it as impossible "fiction" and entertainment. And it's a lot cheaper than "hot" war.
The best prevention of abuse of power is to never grant or vest it in the first place, especially in one or a very limited number of people.
Vesting too much unnecessary power in the hands of a single individual simply invites corruption - we are sort of really "lucky" that Trump is so casual, open and transparent about his own - he endlessly speculates about what he can get away with (shooting and killing someone on Fifth Avenue, pardoning himself, infinite unchecked powers etc.) - because he was so spoiled rotten and corrupt since he was a child that it's absolutely natural to him.
The more he gets away with, the more he will test the "system" which has been long broken and made ever more corrupt (even before Kennedys, Nixon and Clintons) - it's just that every time it's "tested" and cracked open a little more we exhale with a sigh of "relief" and say "the system survived, therefore system 'works' and is strong" - when it's not and it should have been fixed, but the partisan interests on both sides (I'll pretend that there are only two "sides" for practical purposes) prevent that because they want to use the "precedent" for themselves when they get the power to exploit the "system" (whataboutism.) In the immortal words of Fiona Hill, "And here we are!"
As to the slightly paraphrased question in the headline ("What if he does it again?") first, of course, he will - he does it every day, before, after and during his tweet-streams-of-consciousness; and second, why are they asking this question now? The investigation(s) and the threat of impeachment (just like all other kinds of threat) are / were stronger than the impeachment process itself which has already been mocked by McConnell and Trump (in fact he kept asking for it and tried to induce it faster, as the "immunization shot" - so what the hell was the rush? Nobody of any consequence from the House is running in Democratic primaries, so keeping the legal pressure and legal options of subpoenas and expedited court decisions was a no-brainer. But now, Pence rejects calls to declassify new impeachment testimony calling the request illegitimate because the impeachment inquiry has concluded. The vice president's lawyer says it "serves no purpose." Can you blame him? If it's possible to imagine Mike Pence laughing, it may be one of those moments.
With Democrats kneecapping themselves in a "rush to impeachment on narrow grounds" Agent Orange may feel good about himself and feel invincible, so the headline question - "What if" should be changed to "What will happen when...?"