Oh I quite agree with you on that. Democrats are quite good for recruiting wave insurance candiates, but it would need a wave for democrats to win something else than NC or CO. And I don't expect 2020 to be a democratic wave at all.
I don't think 2020 will be a D wave either, but Daines (and Sullivan, I guess) could certainly lose even in a neutral or even a good Republican year. The margins in these two states will probably be closer in 2020 as well. Also keep in mind that Bullock won by 4 while Trump won by 20. The same could definitely happen again.
Again, the ethics of Trump is yet to be determined by the FBI investigation, and the GOP is assuming that Trump is innocent. Let's not rush the investigation, the Democratic maps are fairly Democratic friendly right now, due to the Trump ethics.
Dems will be also looking at GA, with Ossoff, KY with McConnell and Susan Collins in ME, as well as Joni Ernst, aside from Daines and Sullivan.
You really think they'd go after McConnell? Sure, he's unpopular, but KY is trending Republican fast (along with Iowa, for that matter), and would have little chance of being defeated barring a HUGE wave.
