It's not just that there will never be a conviction in the Senate. It's that the acquittal will play straight into the hands of the MSM and the other left-wing agit-prop outfits pimping for Biden and the democrats. Americans would be deluged with the narrative that the acquittal represents a complete vindication of Biden, not just that it represents the depths of political partisanship in Congress.
And more strategically: what happens if, mirabile dictu, there is a conviction? Biden is removed, Harris becomes president, and the democrats then have the votes to put into the VP office one of their hard-left members, someone who could never in a million years get elected to office. From there it's a short hop, skip, and a jump to, say, President Newsom of California.
So, the choices are these: (i) impeach Biden and suffer an acquittal - Biden is vindicated and his re-election chances, and the election chances of democrats generally, go up, or (ii) impeach Biden and win a conviction - Gavin Newsom, or some other noisesome hard-left prog/lib, becomes VP and, in short order, president.
Yeah, I'd prefer not to start up that road in the first place.