I think you're all missing the bigger picture. Yes, it's pretty likely that enforcing party discipline the way Republicans did will cost us seats in the short term, but guess what, Republicans did that, and now they hold majorities in both houses and do basically what they want with it.
I think you're overstating the effects of what the GOP's discipline The party in power tends to do poorly in secondary elections, whether it's midterms, provincial elections, or the European parliament. The GOP was always going to do well in 2010 and 2014, given the maps they faced, the economic situation in 2010, the 2nd term incumbent in 2014 etc. Likewise, the Democrats ought to do very well in 2018.
The question then becomes, how did the GOP (or any party out of power) relative to expectations? The effects of discipline and the Tea Party are far more ambiguous when viewed through this framework. In the House, it looks quite effective. In the Senate, not so much. Betsy DeVos for example, wouldn't have required Mike Pence's vote if the GOP hadn't choked in Indiana and Missouri in 2012.
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