You said that we contractors don’t accumulate (积累) as FTE do.
I do agree that after initial 2Y of tough learning, some FTE could reap the (monetary) rewards whereas consultants are often obliged (due to contract) to leave the team. Although there are long-term contracts, they don’t always work out as promised.
My experience — I stayed in GS for 2.5Y. My later months had much lower "bandwidth" tension i.e. the later months required less learning and figure-things-out. Less stress, fewer negative feedbacks, less worry about my own competence, more confidence , more in-control (because more familiar with the local system). If my compensation becomes 150k I would say that money amounts to "reaping the reward".
As a developer stays longer, the accumulation in terms of his value-add to the team is natural and very likekly . Managers like to point out that after a FTE stays in the team for 2Y her competence, her design, her solutions, her suggestions, her value-add per year grows higher every year. If her initial value-add to the company can be quantified as $100k, every year it grows by 30%. Alas, that doesn’t always translate to compensation.
That’s accumulation in personal income. How about accumulation in tech skill? Staying in one system usually means less exposure to other (newer) technologies. Some developers prefer to be shielded from newer technologies. I embrace them. I feel my technical accumulation is higher when I keep moving from company to company.
 There are exceptions. About 5% of the old timers are, in my view, organization dead weights. Their value-add doesn’t grow and is routinely surpassed by bright new joiner within a year. Company can’t let them go due to political, legal or ethical reasons.
You said IV questions change over time so much (ignoring the superficial changes) that the IV skills we acquire today is uesless in 5Y and we have to again learn new IV skills. Please give one typical example if you can without a lot of explaining (I understand your time constraints). I guess you mean technology churn? If I prepare for a noSQL or big data interview, then I will probably face technology churn.
On the other hand, in my experience, many interview topics remain ever-green including some hard topics — algorithms (classic algos and creative algos), classic data structures, concurrency, java OO, pass-by reference/value, SQL, unix commands, TCP/UDP sockets, garbage collection, asynchronous/synchronous concepts, pub/sub, producer/consumer, thread pool concepts… In the same vein, most coding tests are similar to 10 yeas ago when I first received them. So the study of these topics do accumulate to some extent.