It’s not “some rumour”, it’s a literal and clear threat that was made - even openly by some senators - Joni Ernst for example.
And I don’t think they’re actually “naive” to believe the Republicans won’t do that, given they seem to have made a specific deal not to call witness, and because what the Republicans were planning to do was pretty serious and hard work. They could basically prevent anything from happening that needs Senate approval.
They made a political calculation. Whether it was the right one or not remains to be seen, but dismissing it as “some rumour” is not right. The political calculation was basically as follows:
There is literally no chance of conviction. The argument that the Senate can’t impeach ex-presidents is too good an excuse. It’s bullshit, and means future presidents and federal officials could go on utter rampages in their last few months/weeks, but still, it clearly worked. So that was off the table.
If they brought witnesses, they could probably damage Trump further, and potentially damage a small number of Republican senators.
However, the Republicans would then ensure that they caused maximum damage to them in the Senate, and because of the way that works, that would be quite a lot. It would embarrass the Republicans a bit, but they could probably keep digging up specific excuses to minimize public perception that they were just being jerks. In particular they could make repairing the damage Trump did to various positions very hard. It would also make them look weak/bad.
So the calculation is, is the extra damage they’d do to Trump and those Senators worth the trouble the Senate Republicans could do? And can the Senate Republicans be trusted to keep their word not to do that?
You will note that the Democrats could have done the same when in minority, but historically, they have not, beyond attempting to block specific individuals on rare occasions. This isn’t because they’re saints or something, it’s because it’s hard to do and politically costly, and makes you look bad as well as the other guys. But they could have. It’s just not the usual thing to do though.
And the leader of the Senate Republicans is Mitch McConnell. McConnell hates Trump these days. He’s been extremely clear about it. He voted to acquit for the same dubious “but he’s not the president” reason, but he also made a statement condemning Trump, and saying he was responsible and that the “not president” thing was the only thing in the way. He’s the one who is in a position to hold Senate Republicans to any deal, and it is likely he’ll stick to a deal.
So again the question is, would the extra damage they’d do be worth it? I think almost everyone agrees that Trump is a spent force. The senators in question, even if they ended up damaged, would likely have been replaced once more be Republicans - none were in seats where even if they’d had to resign, the Democrats stood a chance. Would there be long-term benefits to US democracy from doing it? That’s a maybe.
So I can certainly see it as cowardly, and possibly a miscalculation, but I don’t think it was ever going to be hugely successful.
As one of the papers pointed out, the Democrats had already made a key error by focusing on incitement to riot - if they’d focused on Trump attempting to use intimidation, it would have been a lot easier to prove. But even then, the whole “he’s not president, we can’t impeach him” thing remains effective as a defence.