My opinion will probably come across as rather negative on this front, but I don't think SC2 is a good example of 'skill' in video games.
As a background, I was ranked in WC3:TFT 16th and 23rd on two different years in NA on the ladder. I wasn't good enough to go 'pro' but I did regularly play against/with them. I've played a lot of RTSs, and know them inside and out.
The question is, what 'skills' does SC2 actually require? And for the most part its simply clicking speed. Higher APM = higher ladder ranking in SC2. There's very little strategy. Everyone is building the same units in the same order, the same buildings is the same place. The 'optimal' build orders for any given strategy are well known by anyone in gold+, and there are only a handful of viable strategies, deviate from the standard builds and you get rolled.
A metric I like to use is: strategy diversity. A well designed game, will require more complex and more diverse strategies (both in attacking and defending) as you increase the difficulty. A poorly designed game will do the opposite.
Consider chess, playing against your friend you only have to think 1-2 turns in advance and you wipe the floor with them. Grand Masters on the other hand are thinking many more turns in advance, with many more units being utilized (ie. beginners focus on their queen, masters are using every single piece, etc...). There's a real complexity there.
Same goes with real sports. Watch a minor/little league soccer match or hockey game and compare that to the pros, and the complexity and fluidity will be night and day, there's no mistaking one for the other.
Compare that to SC2. Watch a few gold/plat level players, and then watch some pro games. It'll be very hard to spot the difference. The top guys aren't utilizing more complex strategies, they're simply clicking faster. They're still going MMM, they just have 5 more marines at the 10min mark than say the gold/plat player. The gold/plat zerg player will have just as many hives but will have a few less larva and fewer creep tumors.
This is something that the average player does not understand, is not immediately intuitive, and is something that a game designer has to design for. Strategic diversity is a difficult, but rewarding, path. I find (in particularly lately) most game companies stick to having 2 or 3 optimal builds, and simply focus around balancing that.
So that said to your questions, there are two ways I see designers commonly approaching the problem of skill levels and newbie retention:
1st is to take the high road. Generally speaking players don't mind losing if they don't feel cheated, and there is clear and obvious paths for progression. If its clear to the player what and why things went wrong such that they can attempt to correct/improve, losing rarely becomes rage inducing (Atlas Reactor is a good example of this).
2nd and sadly (at least to me) the easier way to ensure noobie retention is to have a high enough RNG factor that noobies will accidentally win from time to time regardless of their skill level (LOL is a perfect example of this). A good-ole skinners box attached to a ladder is more than enough to masquerade as skill for most players; and I can't entirely fault the designers for taking the easier route given how much easier it is to do.