No need for a better design. Keep the skill decay. Just slow it down to a reasonable pace. Let's say an average player spends 4 hours in-game on a weekday, and 6 hours per day in-game on a weekend - so 32 hours a week. Now, does losing a skill point in every unlocked skill every 2 weeks sound reasonable enough? This means that we need to lose 100 of 0.01ths over 64 hours, or 3840 minutes, of playing. 3840/100 = 38.4 minutes. So, we'd lose 0.01 of every unlocked skill every half an hour. That is enough to both bring us back to the skill lot from time to time and let us keep the liberty of engaging in other activities!
Now, if we want our skills to decay faster the higher they are, we can calculate the decay that has to occur by multiplying 0.01 by a rounded sp/5 (sp for skill points). Then, every 30 minutes we lose
0.04 in every unlocked skill that has 18, 19, or 20 points;
0.03 in every unlocked skill that has 13 to 17 points;
0.02 in every unlocked skill that has 8 to 12 points;
0.01 in every unlocked skill that has 3 to 7 points;
none in every unlocked skill that has 0 to 2 points.
(Obviously, the numbers I used for hours are merely off the top of my head, I do not have the actual statistics. If you do, plug in your numbers!)
At the moment, we, the players with much more than 50+ skill points, are losing our skill points faster than gaining. I am not sure how other sims who have got 50+ points feel about it, but I do not appreciate that the game artificially forces me to keep the pace it chose, when it comes to my skilling. It reminds me of that first grade teacher from To Kill A Mockingbird who scolded the student after she discovered that the student was writing, and her argument was: "We don't write in the first grade, we print. You won't learn to write until you're in the third grade".
Yes, it is better that we have a new skill lock every 7 days instead of 21, but it is still a limit. After I get that new skill point in a first day, during the following 6 days, I am forced to either grind endlessly just to keep the skills I have at the same level - or I am forced out of the skill lots, because there is no point in getting the next skill point until these 6 days pass and I get a new skill lock.
... Also, the current skill decay pace gives the unfair advantage to older players over newer players. Just because the newer ones were unlucky to learn about/get interested in TSO later than older players, they are punished with close to never being able to catch up with those who play it for longer. Well, maybe in 2 years, when they too get that 120th lock, but I bet most of them will lose interest in the game earlier that that.