Imagine how American history would have been different if Bill Clinton had actually been removed from office in 1998 or 1999. Al Gore would have gone into the 2000 election as the incumbent, riding a wave of good will, with the country prosperous and at peace. The Clinton legacy would have been far less of a problem for him. He probably would have won easily.
And that points out the difficulty with impeachment under normal circumstances: why would you, as the opposition party, want to replace the incumbent with someone of similar ideology, and less baggage?
These are not normal circumstances. Trump presents a danger to the country; Mike Pence is a generic conservative Republican. Both the Democrats and the Republicans in Congress would undoubtedly prefer Pence to Trump. It is not absurd, then, to think that the GOP might suddenly decide to be shocked by Trump’s shenanigans a few years into his term if his poll numbers look bad, and take action on it.
That, presumably, is the reason that bookies in the UK are giving more or less even odds on Trump leaving office before his term expires.