Merc haven't really run the soft tyres all through testing. Not on longer runs anyway, so I don't think we really know if this problem has gone yet.
Almost no teams have run softs on long runs, almost all the race sims up to the last couple days have been all on hards/mediums, all of Webber, Button, Perez, Massa race sims last week in Barca weren't on softs. THe only real difference is every other team had tried softs at some stage most days, between 3 lap quali runs to test speed, and a few tests for short runs.
ACtually, I think two of the race sims last week, Grosjean and Webber maybe, went with softs for the start, but they both ended up doing like 5 laps then pitting. They could have been using scrubbed softs, because frankly that is a better "race sim" as most will start on softs. But cold weather, poor track conditions and graining, no one did anything meaningful.
Overall the conditions just mean the data on softs is useless, they grain for 2 laps, get up to temp, and are then rubbish. In testing when laps is the most important thing, softs were meaningless.
Considering also most teams only brought their melbourne packages for today and tomorrow, ultimate speed testing on a package you won't use is a waste, while engine, gearbox, electric reliability testing the package doesn't really matter, nor does fuel tests, and loads of other stuff.
Today Hamilton had a crapload of spare softs as they'd used almost none, Rosberg will have the same tomorrow, 5 lap stints, same stint over and over so they can compare settings, on the final race package(or near enough).
When we get to Aussie, the heat, the final setup, we'll see how everyone is on softs, up to now almost no one did any useful long run data on softs. Its also worth pointing out the Merc's were crap on any tyre, they weren't brilliant on the hards, didn't matter what they used, most races they could look decent and competitive, with Schumi often passing several people on early laps, and they'd hold on to faster cars for 7-8 laps then 5 laps before everyone else you'd see them sliding around like crap and pitting early or dropping places constantly.
But they did this with most of their stints, on harder tyres, and in every race, even when softs weren't available. The car was bad for tyre deg, full stop, not just tyre deg on softs and the really impressive thing is the overall speed of the car on the longer runs looks better than everyone else. Though I can't remember maybe useful race sims, they'd do 15-20 laps, and seemingly have less drop in times and a more competitive stint than everyone else. Sure it could be 20 lap stint and 20 laps of fuel, but those times were still noticeably faster than anyone else on their last stint of a race sim, thats where it became impressive. Very few cars could do 20 laps without significant time loss, 3-4 seconds, most were seeing maybe 1.5-2.5second time loss over 12-15 laps and Merc seemed faster and less deg than most.
Simple fact is a car that can go faster overall, can go slower and save tyres better than another car struggling for pace that can't ease off as much. Much of Red Bull's success has been a car cruising around leading to less engines failures, less reliability issues and less tyre wear. Aside from Merc looking good on cold runs, Hamilton's times over last years Merc were pretty amazing, he poled last year on 1.21.7 for Mclaren but Merc were a further 1.3seconds behind that in quali, so the Merc itself has made up a shedload of time.
