Just because the end-to-end service takes longer doesn't necessarily mean that average passenger's journey is also longer. XC's traffic is largely made up of shorter journeys, so whilst those making long journeys constitute the minority of overall passenger numbers.
As for reasonable alternatives/competition, it's half a dozen of one and six of the other. Both routes have sections with no reasonable alternative (between Bristol and Birmingham on SW/NE, between Oxford and Banbury on S/NW) as well as others with plenty of alternatives (between Penzance and Bristol, or Coventry and Stafford).
It's not a competition for who is worst off, nor is GWR/XC/the world out to get Plymouth. Everyone is being affected in some way by these cutbacks.