I moved to Telstra 7 years ago or so because:
A) Coverage was absolutely rubbish on every other carrier. (especially data coverage on the train from Blacktown to the city - you'd very frequently lose internet, usually super slow when connected).
B) Support was utterly woeful on every other carrier (bills, plans etc would be screwed up from time to time, through no fault of ours, and there were long hours fighting with incompetent call centre drones with sometimes very poor manners, who often hung up on you after a 20 minute wait and were often powerless to help you anyway).
I still need good coverage (especially for fast data) and decent service. I'm wondering if the situation in Australia has improved to the point where I can now get those for less than $50 per month for a minimal talk/text/1.5GB-data plan.
While I'm a stingey ozbargainer at heart, I can afford to pay for Telstra if I'm forced to. Being unable to work on the train would cost me more than whatever I'd save.
Have any other carriers stepped up to deliver adequate coverage and service in the last few years?
It all depends on how close you are to the towers. If Optus and Vodafone towers are too far away, then you can use a Telstra reseller like Boost.
Telstra generally have more towers, but it really depends on where you are. Have a look at the coverage maps to see if they're within your expected journey.
Telstra coverage map
Optus coverage map
Vodafone coverage map
Usually the edge of the coverage means it'll be poor service. Check with the providers if they'll have a money back guarantee if the coverage does not suite your expectations.
In regards to the support for different carriers, have a read of Whirlpool to see issues.