Kind of like Johnson County, KS, it was actually a really Republican place down-ballot until incredibly recently, and then had a very strong and visceral reaction to Trump. As late as 2016, Toomey did better there than he did statewide, and there were statewide candidates down-ballot who lost but still carried Chester County.
In the 2018-and-later alignment, it's one of just five totally safe counties for Democrats (together with Philly, DelCo, MontCo, and Allegheny), but it stands out for just how recently it was a nationally Republican place. (Though DelCo, too, was locally extremely Republican until the 2018 cycle, but it had stopped voting Republican nationally for a long time.)
Back in 2012, Republicans were stronger with white voters with higher education, having dominated this voting group in the 2010 and 2014 midterm elections. Additionally, Republicans still held a partisan advantage in terms of voter registration back then; it was only in the last few years that Democrats overtook Republicans in that regard.
You almost understate things; while Republicans suffered a huge collapse with this group between 2004 and 2008, for most of the Obama era these people were outright trending right, and 2014 was actually a
very strong performance. (Notably, compared to Trump, 2014 also saw a strong Republican trend in farming and logging areas,
but very much not industrial ones, which if anything trended anomalously leftwards. The interesting thing about Trump is that most of his gains, apart from some very specific ones, were kind of baked in, while most of his losses were actually extremely contingent.)
