It's both. George is legitimately a good driver, with his talents being masked by that Williams. But at the same time, the Merc is the best car out there. You have Lewis + Merc and you have that 'match made in heaven' scenario. But you can't say Merc are only winning just cuz of Lewis or say Lewis is only winning cuz of the Merc. Saying how good the Merc is doesn't and shouldn't take anything away from Lewis. The problem is too many ppl seem to see it as "complimenting the Merc = disrespecting Lewis". That's entirely the wrong way to look at it. If anything, saying it's all down to Lewis is a bit disrespectful to the thousands of Merc staff responsible for building that car. Lewis, like Max, are able to extract 100% out of their cars.
As for guys like Alex, I think it's down to driving style and preference. Max drives a certain way and when you're their #1 driver, RBR will always develop the car to Max's preference. Meanwhile, Alex will continue to struggle more and more.