Yes, but remember that Edinburgh alone has a population similar to that of the entire Highlands and Islands region. Ullapool has a population of 1.5k, and if you added the populations of everywhere twenty miles from the route of the new line, you'd come to less than 20k. It's such a basket case that the Victorians didn't even get around to building it.
The Port Road, pretty much the same. It's slightly better, but you're talking percentages of percentages for the BCRs for either of these two routes.
Unfortunately, East Kilbride is forever destined to be at the end of a branch line. If you contacted it to Hamilton, you're demolishing a lot of buildings in Blantyre. If you connect it to the main line at Uddingston, then yes, I guess you could run direct EK - Edinburgh trains, but I'm not sure the good people of the Shotts line would be happy for an extra forty minutes added to their trains into Glasgow every day.
Either way, you're building a lot of new line, buying and demolishing a lot of buildings in EK, and the simple answer is why would you? EK doesn't really have a bad deal, it's got dedicated, direct, frequent and soon to be electric trains to Glasgow Central, that take about half an hour. Sure, you could run a train that takes twenty minutes to Motherwell, or an hour and a half to Edinburgh, but that's not where most people want to go.