Glasgow to London doesn't struggle to compete with a flight to Heathrow - there are plenty of people on the trains. The fact that there are some passengers for who flying is preferable and some for who Heathrow is more convenient doesn't alter the fact that for a lot of passengers the train is the preferred option even when it's more expensive.If an argument can be made for Aberdeen-Penzance, or even Edinburgh-Bristol, it can also be made for connecting distant cities that are not already connected too. Aberdeen-Ipswich or Norwich-Swansea. The issue is that there is very little passenger numbers for those journeys and if Glasgow-London struggles to compete with a flight in to Heathrow, a train from Aberdeen, Edinburgh or even Newcastle to Bristol is not going compete with air travel.
If we're talking about say Scotland to the Southwest then the flight will probably be faster, but there's unlikely to be more than one flight a day, if that, on each route. Even before Flybe went bust, Exeter to Edinburgh was only once a day on a stupid little whirly-copter, and was rarely as cheap as the train. Far preferable to get the train at the most convenient time, where I can do a few hours work and then tuck into a four pack while I watch a little Netflix.
I'm not sure where Ipswich comes into this, since neither I or anybody I know has ever wanted to go there. I would point out though that there have been north-east-south-west trains for at least my whole lifetime now, and their existence has at least fed into mine and my family's choice of place to live. Maybe if the train had gone direct to Ipswich that's where my parents would have retired to, but it doesn't so they didn't. Random cuts in long-established services need a good rationale, and some East Anglia-based whatabouterry isn't it.