My point about drivers is that, to resolve the current situation, GWR would have needed to increase the number of drivers about 2 years ago, due to the time it takes to train them.
It's understandable that the link with delayed electrification isn't widely understood. To electrify the railway, Network Rail takes weekend (and increasingly, midweek) blockades. To operate amended timetables, GWR must;
A) Run trains at either end of the blocked section
B) Divert key services where possible, via longer routes
These timetables are inevitably less efficient in terms of resourcing, eating up more crew and units. This is bad enough when agreed and planned at reasonable timescales. Now consider that for months / years now, massively disruptive blocks have been imposed at as little as FIVE DAYS NOTICE. That is so ridiculously close that roster deadlines are missed, so although diagrams are rewritten by Train Planning the crews are (entirely reasonably) entitled to work their agreed rostered hours. On the day, large chunks of the timetable are uncovered. Control will beg and plead for further drivers to come in on overtime to cover the work. Inevitably some things will be cancelled. But the hastily rewritten plan may also have fundamental problems (haste causes mistakes, and planning is a largely manual process), so those crew who are giving up their weekends to work inevitably get caught up in disruption and chaos. If they finish late they might not be legally able to work their booked diagram the next day. So a bad Saturday inevitably means a worse Sunday. Try and recover Sunday and you don't have enough crew for Monday morning. So Sunday is sacrificed to save the Monday morning peak.
The point of this description is to demonstrate how an enormously disruptive engineering project, years late and floundering, but absolutely critical to ensure there is a railway capable of operating the Jan 2019 timetable, impacts on crew resources during a key period when a large number of drivers have to be released for training. If they aren't released then the HSTs gradually go off lease to be replaced by 800s that the drivers don't know how to drive. To date, GWR are just about managing to keep training sufficient crews to run the Mon-Fri timetable. The weekends are carnage, but when you consider everything I've written above, and remember that the electrification shouldn't have overrun into 2018, you'll hopefully see my point.
(Industrial disputes over the new trains also make things worse still, which is one of the reasons Oxford and Reading aren't even being trained yet.)