This is the unstated elephant in the room. If impeachment could not plausibly lead to conviction and removal, there’s a strong argument against it almost irrespective of the underlying crimes and the strength of evidence for them.
But I’d rather see Nadler make this overt and clear: the facts themselves justify and warrant impeachment, but at this stage it seems the GOP-controlled Senate (not to mention a significant subset of the population) would strongly oppose it. That would raise pressure on Trump’s defenders, possibly paving the way for removal.
Another more Machiavellian question would be whether Democrats would rather run against a scandal-plagued Trump and (likely, I would hope) defeat him electorally than impeach him and give the GOP a chance at a do-over. While a lot of focus has been on the Clinton impeachment as a template (Republicans overplaying their hand, and actually losing ground in a mid-term election), it’s also worth noting that Ford came quite close to winning reelection in 1976, despite the stain of Watergate on the GOP, his pardon of Nixon himself, and even a debate gaffe calling Poland a free country,
Ironically, the more damaging to Trump the Mueller investigation is, especially if/as things continue to drip out slowly in periodic court filings, the more Democrats might prefer simply to defeat Trump soundly at the ballot box, rather than risk a Pence-led (or, potentially, some other Republican) party making a competitive election in 2020.