The trial is held in the Senate, and the Senators serve as a judge-less jury.
Partisan impeachment is rightfully difficult, by design. Juries either have to be unanimous or a super-majority, depending on venue. If you can't get a small fraction of the opposition party to agree with the charges, the charges are defective.
If the charges are "here's some crap we scraped together, let's throw it at the wall and see if it sticks", then it deserves to fail. It failed under Clinton and under Trump, partly for partisan reasons but mostly because senators didn't think the charges rose to the level of "high crimes and misdemeanors". Dershowitz has some really good analysis on this.
The founders weren't all convinced that impeachment was even necessary; the president's term is only 4 years. Many were rightfully concerned that impeachment would become a spectacle used by a opposition House to damage the sitting president. And that's what it has become, since the 90's.
No one can preside over a country when any ambitious DA anywhere can drag you into court afterwards. I think the decision today was a good one.
But also think about it this way: no matter how you feel about Trump, imagine how you'd feel if $YOUR_PREFERRED_CANDIDATE was president and lawfare was being conducted against that person by $OPPOSITION_PARTY.
The majority in the court was wise today and closed the door firmly on lawfare as an alternative to campaigning, for all presidents moving forward.